fix(coding-agent): reclaim reused Termux lease pids

[chai1any]

Summary

• refine kill(pid, 0) EPERM handling on Linux and Android with a /proc/<pid>/stat probe
• treat a missing proc entry as a stale or reused lease-owner PID so the session lease can be reclaimed
• retain conservative alive behavior for readable proc entries, other procfs errors, and non-procfs platforms
• add platform-specific liveness regression coverage

On Termux, a dead worker PID can be reused by a SELinux-protected process. The signal probe then returns EPERM while the proc entry is hidden as ENOENT; previously both conditions were treated as proof that the original lease owner was still alive.

Verification

• npx tsx ../../node_modules/vitest/dist/cli.js --run test/session-lease.test.ts (13 tests passed)
• npm run check

Fixes #868

Note

Fix stale session lease detection for reused PIDs on Android/Termux

On Linux/Android, isProcessAlive previously treated EPERM from process.kill(pid, 0) as alive, which caused stale leases to become permanent when a dead worker's PID was reused by a protected process.

• isProcessAlive in session-lease.ts now falls back to reading /proc/<pid>/stat when EPERM is received on Linux/Android; ENOENT in procfs means the process is dead.
• On non-procfs platforms (e.g. Darwin), EPERM continues to be treated as alive.
• The function is now exported and accepts injectable ProcessProbe dependencies for testing.
• Behavioral Change: PIDs returning EPERM on Linux/Android are now considered dead if they have no /proc entry, changing the previous unconditional alive result.

^{Macroscope summarized c30502b.}