fix(merge): gate commit-append separately so declining it keeps hooks

[max-sixty]

Summary

wt merge bundled the project commit-append into the hook-approval batch and shadowed verify = false on any decline. When the project hooks were already approved (and thus filtered out of the prompt), declining the lone commit-append prompt skipped every pre-merge/post-merge hook for that run — even though the user only meant to skip the append. This was flagged as a follow-up during review of #2774.

The append is now resolved through the shared approve_commit_template_append gate — the same path wt step commit / wt step squash already use — so its decline drops only the append and never touches hook approval. Hooks are approved on their own; only a hook decline skips hooks.

Trade-off: when both project hooks and the append are unapproved on a fresh repo, the user now sees two prompts instead of one bundled prompt. This matches the standalone commit/squash flow and is the canonical behavior; the old single-prompt bundling was what introduced the conflation. Already-approved appends still don't re-prompt.

Also in this PR

• Message canonicalization — merge, removal, prune, and switch now print Commands declined, … without hooks, matching the wording step squash / step commit already used. wt hook and wt config approvals add stay bare (they end on decline rather than continuing an operation).
• Docs — the hook Security section now states that declining skips every project command for that operation (already-approved ones included) and that saved approvals are unaffected. Synced to the skill reference.
• Doc-comment fixes — HookGate / PreApprovedGuidance comments no longer describe the removed bundled-prompt flow.

Testing

• New regression test test_merge_decline_append_keeps_approved_hooks: with the hook pre-approved, declining the append still runs the pre-commit hook and the merge succeeds (fails on the old code, passes now).
• test_merge_bundles_append_into_hook_approval renamed to test_merge_prompts_hooks_and_append_separately, updated for the two-prompt flow.
• Four decline snapshots regenerated (message text only).
• Full pre-merge gate green: 3736 tests, clippy, fmt, doc-sync.