fix: friendlier error when watch is already running on port 7734

[greynewell]

Closes #156.

When supermodel is already running in another terminal, the port-in-use error is confusing. Improve the message to tell users their graph is already being kept up to date.

TDD: failing test first.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Enhanced error detection and messaging when the daemon's UDP port is already in use.
  • Improved cross-platform compatibility for handling address-in-use errors across Windows and POSIX systems.
  • Users now receive clearer, more actionable error messages with specific guidance for resolving port conflicts and managing concurrent daemon instances.

[coderabbitai]

Caution

Review failed

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: be974202-4d06-4dd3-8ba8-9a691dd5e857

📥 Commits

Reviewing files that changed from the base of the PR and between ecb93fc and 68ab0a5.

📒 Files selected for processing (2)

• internal/shards/daemon.go
• internal/shards/daemon_test.go

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Walkthrough

The daemon's UDP listener now detects "address already in use" errors more reliably across platforms and returns a friendlier error message directing users to check or stop the other running watcher instance. A corresponding integration test validates this behavior.

Changes

Cohort / File(s)	Summary
UDP Port Conflict Detection internal/shards/daemon.go	Adds isAddrInUse helper function to cross-platform detection of address-in-use errors (handles both POSIX syscall.EADDRINUSE and Windows via error-message substring matching). Run() now returns a user-directed error message when the UDP port is already bound and filesystem watching is disabled.
Port Conflict Test internal/shards/daemon_test.go	New integration test TestPortConflict_FriendlyMessage that simulates UDP port contention by pre-binding a listener, verifies daemon startup fails, and validates the error message contains helpful guidance for the user.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Possibly related PRs

• Fix deadlock when UDP port is already in use #58: Also modifies daemon UDP bind/error handling to special-case EADDRINUSE and changes Run() error messaging when the UDP port is already in use.

Suggested reviewers

• jonathanpopham

Poem

🚨 When ports collide and daemons fight,
A helper function sets things right.
Now users know: "Stop the other watcher!" ✨
No more confusion—clarity matters! 🎯

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/port-conflict-ux

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