fix(dolt): use adaptive ID length instead of hardcoded 6 chars

Port the adaptive ID length algorithm from SQLite to Dolt backend.
The ID length now scales from 3-8 characters based on database size
using birthday paradox collision probability calculations.

- Add adaptive_length.go with length computation based on issue count
- Update generateIssueID to use adaptive length with nonce fallback
- Add collision detection and retry logic matching SQLite behavior

Fixes bd-c40999

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Executed-By: mayor
Role: mayor

internal/storage/dolt/adaptive_length.go

@@ -0,0 +1,129 @@
+package dolt
+
+import (
+	"context"
+	"database/sql"
+	"math"
+	"strconv"
+)
+
+// AdaptiveIDConfig holds configuration for adaptive ID length scaling
+type AdaptiveIDConfig struct {
+	// MaxCollisionProbability is the threshold at which we scale up ID length (e.g., 0.25 = 25%)
+	MaxCollisionProbability float64
+
+	// MinLength is the minimum hash length to use (default 3)
+	MinLength int
+
+	// MaxLength is the maximum hash length to use (default 8)
+	MaxLength int
+}
+
+// DefaultAdaptiveConfig returns sensible defaults for base36 encoding
+// With base36 (0-9, a-z), we can use shorter IDs than hex:
+//
+//	3 chars: ~46K namespace, good for up to ~160 issues (25% collision prob)
+//	4 chars: ~1.7M namespace, good for up to ~980 issues
+//	5 chars: ~60M namespace, good for up to ~5.9K issues
+//	6 chars: ~2.2B namespace, good for up to ~35K issues
+//	7 chars: ~78B namespace, good for up to ~212K issues
+//	8 chars: ~2.8T namespace, good for up to ~1M+ issues
+func DefaultAdaptiveConfig() AdaptiveIDConfig {
+	return AdaptiveIDConfig{
+		MaxCollisionProbability: 0.25, // 25% threshold
+		MinLength:               3,
+		MaxLength:               8,
+	}
+}
+
+// collisionProbability calculates P(collision) using birthday paradox approximation
+// P(collision) ≈ 1 - e^(-n²/2N)
+// where n = number of items, N = total possible values
+func collisionProbability(numIssues int, idLength int) float64 {
+	const base = 36.0 // base36 encoding (0-9, a-z)
+	totalPossibilities := math.Pow(base, float64(idLength))
+	exponent := -float64(numIssues*numIssues) / (2.0 * totalPossibilities)
+	return 1.0 - math.Exp(exponent)
+}
+
+// computeAdaptiveLength determines the optimal ID length for the current database size
+func computeAdaptiveLength(numIssues int, config AdaptiveIDConfig) int {
+	// Try lengths from min to max, return first that meets threshold
+	for length := config.MinLength; length <= config.MaxLength; length++ {
+		prob := collisionProbability(numIssues, length)
+		if prob <= config.MaxCollisionProbability {
+			return length
+		}
+	}
+
+	// If even maxLength doesn't meet threshold, return maxLength anyway
+	return config.MaxLength
+}
+
+// getAdaptiveConfigTx reads adaptive ID config from database, returns defaults if not set
+func getAdaptiveConfigTx(ctx context.Context, tx *sql.Tx) AdaptiveIDConfig {
+	config := DefaultAdaptiveConfig()
+
+	// Read max_collision_prob
+	var probStr string
+	err := tx.QueryRowContext(ctx, `SELECT value FROM config WHERE `+"`key`"+` = ?`, "max_collision_prob").Scan(&probStr)
+	if err == nil && probStr != "" {
+		if prob, err := strconv.ParseFloat(probStr, 64); err == nil {
+			config.MaxCollisionProbability = prob
+		}
+	}
+
+	// Read min_hash_length
+	var minLenStr string
+	err = tx.QueryRowContext(ctx, `SELECT value FROM config WHERE `+"`key`"+` = ?`, "min_hash_length").Scan(&minLenStr)
+	if err == nil && minLenStr != "" {
+		if minLen, err := strconv.Atoi(minLenStr); err == nil {
+			config.MinLength = minLen
+		}
+	}
+
+	// Read max_hash_length
+	var maxLenStr string
+	err = tx.QueryRowContext(ctx, `SELECT value FROM config WHERE `+"`key`"+` = ?`, "max_hash_length").Scan(&maxLenStr)
+	if err == nil && maxLenStr != "" {
+		if maxLen, err := strconv.Atoi(maxLenStr); err == nil {
+			config.MaxLength = maxLen
+		}
+	}
+
+	return config
+}
+
+// countTopLevelIssuesTx returns the number of top-level issues (excluding child issues)
+func countTopLevelIssuesTx(ctx context.Context, tx *sql.Tx, prefix string) (int, error) {
+	var count int
+	// Count only top-level issues (no dot in ID after prefix)
+	// Using INSTR for MySQL/Dolt compatibility
+	err := tx.QueryRowContext(ctx, `
+		SELECT COUNT(*)
+		FROM issues
+		WHERE id LIKE CONCAT(?, '-%')
+		  AND INSTR(SUBSTRING(id, LENGTH(?) + 2), '.') = 0
+	`, prefix, prefix).Scan(&count)
+	if err != nil {
+		return 0, err
+	}
+	return count, nil
+}
+
+// GetAdaptiveIDLengthTx returns the appropriate hash length based on database size
+func GetAdaptiveIDLengthTx(ctx context.Context, tx *sql.Tx, prefix string) (int, error) {
+	// Get current issue count
+	numIssues, err := countTopLevelIssuesTx(ctx, tx, prefix)
+	if err != nil {
+		return 6, err // Fallback to 6 on error
+	}
+
+	// Get adaptive config
+	config := getAdaptiveConfigTx(ctx, tx)
+
+	// Compute optimal length
+	length := computeAdaptiveLength(numIssues, config)
+
+	return length, nil
+}

internal/storage/dolt/issues.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/steveyegge/beads/internal/idgen"
 	"github.com/steveyegge/beads/internal/types"
 )
 
@@ -617,15 +618,47 @@ func markDirty(ctx context.Context, tx *sql.Tx, issueID string) error {
 	return err
 }
 
-// nolint:unparam // error return kept for interface consistency
-func generateIssueID(_ context.Context, _ *sql.Tx, prefix string, issue *types.Issue, _ string) (string, error) {
-	// Simple hash-based ID generation
-	// Use first 6 chars of content hash
-	hash := issue.ComputeContentHash()
-	if len(hash) > 6 {
-		hash = hash[:6]
+// generateIssueID generates a unique hash-based ID for an issue
+// Uses adaptive length based on database size and tries multiple nonces on collision
+func generateIssueID(ctx context.Context, tx *sql.Tx, prefix string, issue *types.Issue, actor string) (string, error) {
+	// Get adaptive base length based on current database size
+	baseLength, err := GetAdaptiveIDLengthTx(ctx, tx, prefix)
+	if err != nil {
+		// Fallback to 6 on error
+		baseLength = 6
 	}
-	return fmt.Sprintf("%s-%s", prefix, hash), nil
+
+	// Try baseLength, baseLength+1, baseLength+2, up to max of 8
+	maxLength := 8
+	if baseLength > maxLength {
+		baseLength = maxLength
+	}
+
+	for length := baseLength; length <= maxLength; length++ {
+		// Try up to 10 nonces at each length
+		for nonce := 0; nonce < 10; nonce++ {
+			candidate := generateHashID(prefix, issue.Title, issue.Description, actor, issue.CreatedAt, length, nonce)
+
+			// Check if this ID already exists
+			var count int
+			err = tx.QueryRowContext(ctx, `SELECT COUNT(*) FROM issues WHERE id = ?`, candidate).Scan(&count)
+			if err != nil {
+				return "", fmt.Errorf("failed to check for ID collision: %w", err)
+			}
+
+			if count == 0 {
+				return candidate, nil
+			}
+		}
+	}
+
+	return "", fmt.Errorf("failed to generate unique ID after trying lengths %d-%d with 10 nonces each", baseLength, maxLength)
+}
+
+// generateHashID creates a hash-based ID for a top-level issue.
+// Uses base36 encoding (0-9, a-z) for better information density than hex.
+func generateHashID(prefix, title, description, creator string, timestamp time.Time, length, nonce int) string {
+	return idgen.GenerateHashID(prefix, title, description, creator, timestamp, length, nonce)
 }
 
 func isAllowedUpdateField(key string) bool {