fix(init): suppress detached-HEAD advice for pinned modules (#50)

[olavostauros]

Summary

Add -c advice.detachedHead=false to the git checkout "\$pin" call in .mise/tasks/init so pinned-module checkout during modules init does not print Git's verbose detached-HEAD advice block.

Also adds a pinned <sha> status line for pinned modules (matching the tracked module format tracking main @ <sha>), keeping output symmetrical and owned by modules.

Tracked modules already use -q (quiet) checkouts and are unaffected.

Design decision

The issue (#50) suggested the output format:

  den: pinned <sha>
  fold: tracking main @ <sha>

This was chosen over two alternatives:

Option	Format	Chosen?
A: minimalist	No status line (just clone line + exit code)	✗ — leaves asymmetric output; no feedback on checkout result
B: symmetric with @	pinned @ abc1234	✗ — tracked format has tracking main @ abc1234 where @ separates branch from SHA; pinned has no branch, so @ reads oddly
C: issue's suggestion	pinned abc1234	✓ — clean, no dangling preposition

Changes

File	Change
.mise/tasks/init	Added -c advice.detachedHead=false to pinned checkout + prints pinned <sha> on success
test/init.bats	2 new tests: (1) verifies no advice block + pinned line present, (2) real checkout errors still surface

Validation

• mise run test — 137/137 passing
• codebase lint "\$PWD"
• git diff --check