fix: bound POSIX installer lock waits

[jamilahmadzai]

Summary

Fixes #413.

A live but wedged POSIX installer currently leaves every later bootstrap blocked in bare flock 3 forever. Since ensure-monk-agent.sh runs from the plugin startup path, this can make all subsequent coding-agent sessions on the host appear hung.

This change:

• bounds the contended installer-lock wait to 60 seconds by default via flock -w;
• supports MONK_AGENT_INSTALL_LOCK_TIMEOUT as a validated non-negative integer, allowing deterministic tests and operator tuning;
• returns a clear non-zero diagnostic when the deadline expires;
• updates all three rendered POSIX copies (root, Claude/Codex plugin, and Antigravity), avoiding the incomplete-copy problem called out on fix: prevent concurrent launcher race in ensure-monk-agent.sh #50;
• adds an actual kernel-lock regression fixture and exercises it on both Ubuntu and macOS CI;
• verifies both sides of contention: a wedged holder reaches the deadline, while a holder released before it lets installation continue normally.

Verification

The new regression first failed on unmodified v0.1.58: its five-second watchdog had to terminate scripts/ensure-monk-agent.sh. After the patch, all three copies exit on the configured one-second deadline and emit the expected diagnostic. A deterministic release-before-deadline case also proves the installer acquires the released lock and returns the managed binary without using the network.

Locally passed:

./tests/ensure-monk-agent-lock-timeout.sh
ensure-monk-agent lock-timeout tests passed.

./tests/start-monk-agent-fastpath.sh
start-monk-agent fast-path tests passed.

./tests/start-monk-agent-readiness-timeout.sh
readiness_timeout_status=pass elapsed=2s sleeps=2

./tests/start-monk-agent-antigravity-python-path.sh
Antigravity Python fallback handles apostrophes in the config path.

Additional checks:

• sh -n on all changed shell scripts
• git diff --check
• byte-for-byte equality of all three shipped ensure-monk-agent.sh copies

Bounty/product-use context

I installed official plugin v0.1.58 through the Monk Codex marketplace, started monk-agent, authenticated the MCP server with the same GitHub-linked Monk account, initialized this repository as a workspace, and successfully called monk.project.analyze. The linked report is therefore based on a live installed product run, with a deterministic concurrency reproduction—not code reading alone.

[jamilahmadzai]

Independent fork CI is green on the current head 4bef4fd: Install E2E run 33336366543 passed Ubuntu, macOS, and Windows. The installer-lock regression passed on both POSIX runners and now covers both outcomes: a wedged holder exits at the configured deadline, while a holder released before that deadline lets installation continue. The upstream run remains action_required only because fork workflows need maintainer approval.

[jamilahmadzai]

Refreshed this PR against current main, preserving the newly added CI cases alongside the lock-deadline regression. All five local POSIX regression scripts pass, and the three shipped installer copies match. The current-head fork Install E2E run passes on Ubuntu, macOS, and Windows: https://github.com/jamilahmadzai/monk-plugin/actions/runs/34205983157. Ready for review; upstream fork-workflow approval may still be required.