test: bring every package above the 80% coverage target

Per-package coverage was below the AGENTS.md target in six packages:

  cmd/ask                  0.0% -> 83.3%
  cmd/hexai-tmux-edit     10.0% -> 93.3%
  cmd/hexai-tmux-action   27.8% -> 95.7%
  cmd/hexai-mcp-server    41.9% -> 88.2%
  internal/taskproxy      61.8% -> 98.2%
  internal/filelock       77.3% -> 100.0%

The four cmd packages each had a main() that mixed flag parsing, struct
construction, and runtime delegation, so nothing called from a test
hit those statements. Each main() is now a one-line wrapper around a
testable runMain(args, stdin, stdout, stderr) int that uses
flag.NewFlagSet (instead of the global flag.Parse) so tests can drive
it repeatedly. The deprecation banner in hexai-mcp-server is now a
package-level constant, kept identical, so tests can assert on it
directly without redirecting os.Stderr.

The internal packages got new tests for paths that were previously
unreachable: filelock's retry-then-success and non-EWOULDBLOCK error
branches, and taskproxy's NewRunner / findTaskBinary / detectRepoRoot /
runTaskCommand helpers (the ones that shell out to git and task).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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