fix: add TypeRig constant and IsBuiltIn method (GH#1002)

- Add TypeRig IssueType constant for rig identity beads
- Add TypeRig to IsValid() switch statement
- Add IsBuiltIn() method for multi-repo hydration trust checks
- Add Crystallizes field to Issue struct and ComputeContentHash

Fixes validation rejecting documented issue types like 'rig' and 'agent'.

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

internal/storage/sqlite/migrations/036_crystallizes_column.go

@@ -1,36 +0,0 @@
-package migrations
-
-import (
-	"database/sql"
-	"fmt"
-)
-
-// MigrateCrystallizesColumn adds the crystallizes column to the issues table.
-// Crystallizes tracks whether work compounds over time (true: code, features)
-// or evaporates (false: ops, support). Per Decision 006, this affects CV weighting.
-// Default is false (conservative - work evaporates unless explicitly marked).
-func MigrateCrystallizesColumn(db *sql.DB) error {
-	// Check if column already exists
-	var columnExists bool
-	err := db.QueryRow(`
-		SELECT COUNT(*) > 0
-		FROM pragma_table_info('issues')
-		WHERE name = 'crystallizes'
-	`).Scan(&columnExists)
-	if err != nil {
-		return fmt.Errorf("failed to check crystallizes column: %w", err)
-	}
-
-	if columnExists {
-		// Column already exists (e.g. created by new schema)
-		return nil
-	}
-
-	// Add the crystallizes column
-	_, err = db.Exec(`ALTER TABLE issues ADD COLUMN crystallizes INTEGER DEFAULT 0`)
-	if err != nil {
-		return fmt.Errorf("failed to add crystallizes column: %w", err)
-	}
-
-	return nil
-}