feat(log): add a rotating log file and show its path in the TUI

[nwebbot]

Why

LogBuffer keeps the process log stream in memory for the TUI panel, but nothing reaches disk. Once the TUI exits the run leaves no record — there is nothing to go back to when debugging a reconnect, a bootstrap, or a crash that happened twenty minutes ago.

Downstream, retask-cli wants exactly this for retask sandbox connect: keep the TUI panel as-is, and additionally append every line to retask.log in the working folder, with the active path visible on the Logs divider.

What

LogFileConfig + OpenLogFile (new logfile.go)

The library still never installs a logger of its own. OpenLogFile hands back an io.Writer the caller tees into whatever handler already feeds the TUI:

cfg := agentfleet.DefaultConfig()
cfg.LogFile.Path = "retask.log"

logFile, err := agentfleet.OpenLogFile(cfg.LogFile)
defer logFile.Close()

logBuf := agentfleet.NewLogBuffer(500)
cfg.TUI.Log     = logBuf
cfg.TUI.LogPath = logFile.Path()

logger := slog.New(slog.NewTextHandler(io.MultiWriter(logBuf, logFile), nil))

Setting	Default	Meaning
LogFile.Enabled	true	Write the log stream to a file.
LogFile.Path	<cwd>/agentfleet.log	Live log file; relative paths resolve against the working directory.
LogFile.MaxBytes	10MB	Rotate once the live file exceeds this. 0 disables rotation.
LogFile.Backups	5	Rotated generations kept. 0 truncates instead.
TUI.ShowLogPath	true	Render TUI.LogPath on the log panel divider.

Rotation follows the Unix convention — the live file keeps its name, older generations shift down through <path>.1, <path>.2, … up to Backups, and the oldest is discarded. A single Write is never split across two generations, so a log line always lands whole in one file. Reopening an existing file appends rather than truncating.

*LogFile is nil-safe on every method and OpenLogFile returns nil, nil when disabled, so turning the file off needs no branching at the call site.

TUI divider

TUIConfig.LogPath / ShowLogPath render the path on the Logs divider:

─ Logs (/Users/bot/retask-sessions/nweb/session-18dcdfe8/retask.log) ───────────────

The path elides from the left as the terminal narrows, and drops entirely when there is no room, so the divider always occupies exactly one row:

termW=44   ─ Logs (…nweb/session-18dcdfe8/retask.log) ─
termW=20   ─ Logs (…task.log) ─
termW=16   ─ Logs ─────────────

Compatibility

Purely additive. Existing callers keep their current behaviour: TUIConfig.LogPath is empty by default, so the divider still renders Logs, and nothing opens a file unless OpenLogFile is called. DefaultConfig() gains the LogFile block and TUI.ShowLogPath: true, following the existing pattern for AutoOpen.

Testing

go vet ./..., gofmt -l, go build ./... and go test -race ./... all pass locally.

New coverage: rotation at the threshold, generation shifting, discard beyond Backups, Backups: 0 truncation, MaxBytes: 0 never rotating, oversized single writes, append-across-opens, absolute path resolution, write-after-close, concurrent writes under -race, nil-writer no-op, plus divider label rendering at wide, narrow and too-narrow terminal widths.

Follow-up

A tagged release is needed before nwebxyz/retask-cli can consume this.

🤖 Generated with Claude Code