Add config-based close hooks (bd-g4b4)

Implements on_close hooks in .beads/config.yaml for automation and
notifications. Hooks receive issue data via environment variables
(BEAD_ID, BEAD_TITLE, BEAD_TYPE, BEAD_PRIORITY, BEAD_CLOSE_REASON)
and run via sh -c.

Changes:
- internal/config: Add HookEntry type and GetCloseHooks()
- internal/hooks: Add RunConfigCloseHooks() for executing config hooks
- cmd/bd: Call RunConfigCloseHooks after successful close
- docs/CONFIG.md: Document hooks configuration with examples

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

internal/hooks/config_hooks.go

@@ -0,0 +1,66 @@
+// Package hooks provides a hook system for extensibility.
+// This file implements config-based hooks defined in .beads/config.yaml.
+
+package hooks
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"os/exec"
+	"strconv"
+	"time"
+
+	"github.com/steveyegge/beads/internal/config"
+	"github.com/steveyegge/beads/internal/types"
+)
+
+// RunConfigCloseHooks executes all on_close hooks from config.yaml.
+// Hook commands receive issue data via environment variables:
+//   - BEAD_ID: Issue ID (e.g., bd-abc1)
+//   - BEAD_TITLE: Issue title
+//   - BEAD_TYPE: Issue type (task, bug, feature, etc.)
+//   - BEAD_PRIORITY: Priority (0-4)
+//   - BEAD_CLOSE_REASON: Close reason if provided
+//
+// Hooks run synchronously but failures are logged as warnings and don't
+// block the close operation.
+func RunConfigCloseHooks(ctx context.Context, issue *types.Issue) {
+	hooks := config.GetCloseHooks()
+	if len(hooks) == 0 {
+		return
+	}
+
+	// Build environment variables for hooks
+	env := append(os.Environ(),
+		"BEAD_ID="+issue.ID,
+		"BEAD_TITLE="+issue.Title,
+		"BEAD_TYPE="+string(issue.IssueType),
+		"BEAD_PRIORITY="+strconv.Itoa(issue.Priority),
+		"BEAD_CLOSE_REASON="+issue.CloseReason,
+	)
+
+	timeout := 10 * time.Second
+
+	for _, hook := range hooks {
+		hookCtx, cancel := context.WithTimeout(ctx, timeout)
+
+		// #nosec G204 -- command comes from user's config file
+		cmd := exec.CommandContext(hookCtx, "sh", "-c", hook.Command)
+		cmd.Env = env
+		cmd.Stdout = os.Stdout
+		cmd.Stderr = os.Stderr
+
+		err := cmd.Run()
+		cancel()
+
+		if err != nil {
+			// Log warning but don't fail the close
+			name := hook.Name
+			if name == "" {
+				name = hook.Command
+			}
+			fmt.Fprintf(os.Stderr, "Warning: close hook %q failed: %v\n", name, err)
+		}
+	}
+}