feat(ai): session harness contract — incomplete, run_id, confirm bind

Add docs/16 for session state machine, incomplete/resume, queue semantics,
and agent-loop/harness/MCP gap review. Wire RunOutcome on max-rounds,
run_id through audit, TrimMessages/Redact hooks, ConfirmBindHash on write
confirm path; session file skeleton without HTTP yet. No push.

server/internal/aitools/audit.go

@@ -15,6 +15,7 @@ type AuditEntry struct {
 	ConnectionID string `json:"connection_id,omitempty"`
 	Mode         string `json:"mode"`
 	Source       string `json:"source,omitempty"` // mcp | agent
+	RunID        string `json:"run_id,omitempty"`
 	OK           bool   `json:"ok"`
 	Summary      string `json:"summary,omitempty"`
 	Error        string `json:"error,omitempty"`