fix(commands): register /caveman-stats so Claude Code doesn't reject it (#470)

[ousamabenyounes]

Summary

Fixes #470

commands/ ships only 4 .toml files (caveman.toml, caveman-commit.toml,
caveman-init.toml, caveman-review.toml). caveman-stats.toml is missing,
so Claude Code rejects /caveman-stats as Unknown command before the
UserPromptSubmit hook in src/hooks/caveman-mode-tracker.js ever sees the
prompt — even though both README.md and INSTALL.md advertise the command and
the hook contains a statsMatch regex that intercepts and injects the stats
output as the hook reason.

This PR adds the missing commands/caveman-stats.toml so the slash command
is registered. The prompt field is /caveman-stats {{args}} so the resolved
prompt (after args substitution) matches the live hook regex and reaches the
existing stats handler.

Test verification (RED → GREEN)

Three regression tests live in tests/installer/slash-commands.test.mjs. They
mirror the live hook regex from src/hooks/caveman-mode-tracker.js so they
stay representative if the hook shifts.

RED (with commands/caveman-stats.toml stashed):

# tests 3
# pass 0
# fail 3
not ok 1 - #470 commands/caveman-stats.toml exists so Claude Code registers /caveman-stats
  error: 'Missing commands/caveman-stats.toml — Claude Code rejects /caveman-stats as "Unknown command" before the UserPromptSubmit hook can intercept (issue #470).'
not ok 2 - #470 caveman-stats.toml declares a non-empty description for the slash-command picker
  error: "ENOENT: no such file or directory, open '.../commands/caveman-stats.toml'"
not ok 3 - #470 caveman-stats.toml prompt is intercepted by the mode-tracker regex
  error: "ENOENT: no such file or directory, open '.../commands/caveman-stats.toml'"

GREEN (with the new toml applied):

# tests 3
# pass 3
# fail 0
ok 1 - #470 commands/caveman-stats.toml exists so Claude Code registers /caveman-stats
ok 2 - #470 caveman-stats.toml declares a non-empty description for the slash-command picker
ok 3 - #470 caveman-stats.toml prompt is intercepted by the mode-tracker regex

Full suite baseline preserved:

• before: npm test → 47 pass / 4 fail (4 pre-existing opencode-installer failures, unrelated)
• after: npm test → 50 pass / 4 fail (same 4, plus the 3 new tests above)

No green test flipped to red.

Files changed

File	Change
commands/caveman-stats.toml	NEW — registers /caveman-stats with description + prompt = "/caveman-stats {{args}}" so the resolved prompt matches the existing hook regex.
tests/installer/slash-commands.test.mjs	NEW — three node:test cases pinning file presence, non-empty description, and prompt-matches-hook-regex for #470.