feat(audit): add append-only agent audit trail (.beads/interactions.jsonl)

Implements audit logging for agent interactions to support auditing and
dataset generation (fixes #649).

New features:
- .beads/interactions.jsonl (append-only audit log)
- bd audit record: log LLM calls, tool calls, or pipe JSON via stdin
- bd audit label <id>: append labels (good/bad) for dataset curation
- bd compact --audit: optionally log LLM prompt/response during compaction
- bd init: creates empty interactions.jsonl
- bd sync: includes interactions.jsonl in staging

Audit entries are append-only - labeling creates new entries that
reference parent entries by ID.

Closes #649

Co-authored-by: Dmitry Chichkov <dchichkov@nvidia.com>

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

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

cmd/bd/init.go

@@ -195,6 +195,16 @@ With --stealth: configures global git settings for invisible beads usage:
 					}
 				}
 
+				// Create empty interactions.jsonl file (append-only agent audit log)
+				interactionsPath := filepath.Join(beadsDir, "interactions.jsonl")
+				if _, err := os.Stat(interactionsPath); os.IsNotExist(err) {
+					// nolint:gosec // G306: JSONL file needs to be readable by other tools
+					if err := os.WriteFile(interactionsPath, []byte{}, 0644); err != nil {
+						fmt.Fprintf(os.Stderr, "Error: failed to create interactions.jsonl: %v\n", err)
+						os.Exit(1)
+					}
+				}
+
 				// Create metadata.json for --no-db mode
 				cfg := configfile.DefaultConfig()
 				if err := cfg.Save(beadsDir); err != nil {
@@ -234,6 +244,16 @@ With --stealth: configures global git settings for invisible beads usage:
 				fmt.Fprintf(os.Stderr, "Warning: failed to create/update .gitignore: %v\n", err)
 				// Non-fatal - continue anyway
 			}
+
+			// Ensure interactions.jsonl exists (append-only agent audit log)
+			interactionsPath := filepath.Join(beadsDir, "interactions.jsonl")
+			if _, err := os.Stat(interactionsPath); os.IsNotExist(err) {
+				// nolint:gosec // G306: JSONL file needs to be readable by other tools
+				if err := os.WriteFile(interactionsPath, []byte{}, 0644); err != nil {
+					fmt.Fprintf(os.Stderr, "Warning: failed to create interactions.jsonl: %v\n", err)
+					// Non-fatal - continue anyway
+				}
+			}
 		}
 
 		// Ensure parent directory exists for the database
@@ -1555,7 +1575,6 @@ Aborting.`, yellow("⚠"), dbPath, cyan("bd list"), prefix)
 	return nil // No database found, safe to init
 }
 
-
 // landingThePlaneSection is the "landing the plane" instructions for AI agents
 // This gets appended to AGENTS.md and @AGENTS.md during bd init
 const landingThePlaneSection = `
@@ -1608,17 +1627,17 @@ func updateAgentFile(filename string, verbose bool) error {
 		// File doesn't exist - create it with basic structure
 		newContent := fmt.Sprintf(`# Agent Instructions
 
-This project uses **bd** (beads) for issue tracking. Run ` + "`bd onboard`" + ` to get started.
+This project uses **bd** (beads) for issue tracking. Run `+"`bd onboard`"+` to get started.
 
 ## Quick Reference
 
-` + "```bash" + `
+`+"```bash"+`
 bd ready              # Find available work
 bd show <id>          # View issue details
 bd update <id> --status in_progress  # Claim work
 bd close <id>         # Complete work
 bd sync               # Sync with git
-` + "```" + `
+`+"```"+`
 %s
 `, landingThePlaneSection)