Fix shellper processes dying when Tower restarts (#324)

[waleedkadous]

Summary

• Redirect shellper stderr to a log file instead of a pipe, eliminating the parent→child pipe dependency that caused shellper crashes on Tower exit
• Add defensive error handlers on process.stdout/process.stderr in shellper-main to swallow EPIPE/EBADF errors
• Add regression test verifying shellper survives parent exit with both fix approaches

Root Cause

Shellper was spawned with stdio: ['ignore', 'pipe', 'pipe']. The stderr pipe connected shellper fd 2 to Tower. When Tower exited via process.exit(0), the pipe broke. On macOS, process.stderr connected to a pipe uses async writes — the resulting EPIPE errors surfaced as unhandled 'error' events on the stream, crashing the shellper process.

Changes

session-manager.ts:

• Changed shellper spawn to use a file FD for stderr (socketPath.replace('.sock', '.log')) instead of 'pipe'
• Added cleanup of .log companion files in unlinkSocketIfExists and cleanupStaleSockets
• Added explicit exit log message (previously bundled inside logStderrTail)

shellper-main.ts:

• Added stream.on('error', () => {}) handlers on process.stdout and process.stderr at startup as a defensive measure

shellper-survive-parent-exit.test.ts (new):

• Test 1: Spawns shellper with file-based stderr, verifies it stays alive and writes to log file
• Test 2: Spawns shellper with old pipe-based stderr, destroys the pipe, verifies shellper survives thanks to error handlers

session-manager.test.ts:

• Updated 3 stderr tail logging tests to reflect new behavior (stderr goes to file, not pipe)

Test plan

• All 211 terminal tests pass (0 failures)
• Regression test: shellper with file stderr survives parent exit
• Regression test: shellper with pipe stderr survives broken pipe (error handler)
• TypeScript compiles cleanly
• Manual verification: restart Tower and confirm shellper processes survive

Closes #324

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