Fix ~50 errcheck warnings across the codebase:
- Add explicit `_ =` for intentionally ignored error returns (cleanup,
best-effort operations, etc.)
- Use `defer func() { _ = ... }()` pattern for defer statements
- Handle tmux SetEnvironment, KillSession, SendKeysRaw returns
- Handle mail router.Send returns
- Handle os.RemoveAll, os.Rename in cleanup paths
- Handle rand.Read returns for ID generation
- Handle fmt.Fprint* returns when writing to io.Writer
- Fix for-select with single case to use for-range
- Handle cobra MarkFlagRequired returns
All tests pass. Code compiles without errors.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Implement GenerateMRID(prefix, branch) to generate merge request IDs
following the convention: <prefix>-mr-<hash>
- Hash derived from branch name + timestamp + random bytes for uniqueness
- Example output: gt-mr-abc123
- Includes deterministic variant for testing (GenerateMRIDWithTime)
Closes: gt-h5n.2
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
