feat: persistent error logging for hook events

[scotthavird]

The bug

Hook send-event failures were silent. fileLog (cmd/hook.go) early-returned unless debug: true was set, the async subprocess set stderr = nil (internal/client/api.go), and even the existing debug-only log went to /tmp/promptconduit-hook.log — a path that's blown away on reboot and confusingly collides with the macOS app's own log file.

Net effect: when events stopped reaching the platform (network blip, expired key, server 5xx), the user had no on-disk trace to diagnose from.

The fix

New internal/logger package:

• Error(format, args...) — always written, regardless of config.
• Debug(format, args...) — only written after SetDebug(true) (the hook entry points call this from the loaded config).
• Lives at ~/.config/promptconduit/logs/promptconduit.log (XDG-aware).
• Rotates at 1 MiB to a single .log.1 backup; total footprint ≤ 2 MiB.
• Standalone — no dependency on client package, so it's safe to call from anywhere including the hot hook path.

Wiring:

• cmd/hook.go: every failure path now calls logger.Error; trace lines use logger.Debug; legacy /tmp/promptconduit-hook.log writer removed.
• internal/client/api.go: the async subprocess now inherits a fd pointing at the log file as Stderr, so a panic that escapes the in-process logger still leaves a trace.

Plus a new promptconduit logs command:

promptconduit logs              # last 50 lines
promptconduit logs -n 200       # last 200 lines
promptconduit logs --follow     # tail -f (uses /usr/bin/tail when available)
promptconduit logs --path       # just print the path
promptconduit logs --clear      # truncate

Verification

End-to-end on macOS:

Success path (debug=true config):

DEBUG pid=84525 Hook started
DEBUG pid=84525 Detected tool: claude-code, hook event: PreToolUse
DEBUG pid=84537 Async subprocess: envelope sent successfully

Failure path (bad API URL, debug=false in config):

ERROR pid=85171 Async subprocess API send failed (url=http://127.0.0.1:1): dial tcp 127.0.0.1:1: connect: connection refused

Exactly one ERROR line, zero DEBUG noise. Goal achieved: silent failures are now loud, verbose tracing stays off unless explicitly enabled.

Test plan

• go test ./internal/logger/ — 5 tests pass (Error always writes, Debug gated, rotation at MaxBytes, Tail returns last N lines, Tail handles missing file)
• make test — full suite green
• go vet ./... clean
• Real hook with successful API send → DEBUG breadcrumbs in log
• Real hook with unreachable API + debug=false → exactly one ERROR line in log
• promptconduit logs --path, --clear, -n, --follow all work
• Windows: the --follow fallback to polling kicks in when tail is absent

🤖 Generated with Claude Code