fix: remove unused variable and dead code warnings

[Ziinc]

This PR cleans up unused code in the codebase by removing unused parameters and eliminating dead code.
Key changes:

• Prefixed unused operation parameters with underscore in IdempotencyStore methods (reclaim_stale, claimed_at, complete, and abandon) to suppress compiler warnings while maintaining the method signatures for API compatibility
• Removed the unused enabled() function from feature_preview.rs command module, which was a wrapper around feature_preview::is_enabled() that was not being called anywhere
  These changes improve code cleanliness by eliminating compiler warnings and removing unreachable code without affecting the public API or functionality.
  https://claude.ai/code/session_016cWEgf7mftVW9Nex9sxvN8

[Ziinc]

verify / verify / ubuntu-22.04 is failing on the max-lines lint rule for test/integration/workspace/create-pr.test.tsx (563 lines, limit 500). This file isn't touched by this PR's diff (only src-tauri/src/core/idempotency_store.rs and src-tauri/src/commands/feature_preview.rs change here), and the same failure reproduces on main at HEAD (5396f43) — confirmed by checking the file there directly. No fix PR exists yet for it. Re-running this job won't help since it's a deterministic lint failure, not a flake, so I'm not spending the one re-run on it.

---

Generated by Claude Code

[github-actions]

🚀 Web preview: https://preview-447.treq-9zy.pages.dev
📊 Lighthouse report: https://github.com/treq-dev/treq/actions/runs/34514031017/artifacts/10167303379

[Ziinc]

js / test:integration / ubuntu-22.04 is failing on test/integration/filepicker.test.tsx > opens via Ctrl+P, shows initial state, searches files, and selects a result — a 5000ms test timeout. This test file isn't touched by this PR's diff, and the same test failed identically (same line, same timeout) on both the pre-merge run and the post-merge run of this PR, so it isn't a one-off flake tied to my changes. No fix exists for it yet upstream. Not spending the re-run here since it already reproduced twice independently.

---

Generated by Claude Code

[Ziinc]

rust / test / windows-latest is failing on 5 Windows-only tests: commands::pty_commands::tests::resolves_workspace_id_to_absolute_working_directory and 4 jj::tests::reconcile_* cases. None of these touch the code this PR changes (idempotency_store.rs, feature_preview.rs), and the same 5 tests failed identically (same names, same failure) on both the pre-merge and post-merge CI runs of this PR — so it isn't a flake introduced by this change. No fix exists for it yet upstream. Not spending the re-run since it already reproduced twice independently.

---

Generated by Claude Code