fix: use builtin cd in fish shell integration

[worktrunk-bot]

Problem

wt switch <branch> reported a successful switch but then failed to change directory under fish when the kidonng/zoxide.fish plugin is active, printing zoxide: no match found / Error: Directory not found even though the worktree exists (#3159).

The fish integration changed directory with a bare cd -- "$target". zoxide.fish replaces cd with a zoxide query function. With cd -- "$target" it receives two arguments (-- and the path), so its single-directory fast path (argc -eq 1) is skipped and it falls through to fuzzy-query mode, which finds no match for -- /path/to/worktree.

The bash and zsh wrappers were already immune because they use builtin cd; only fish used a bare cd.

Solution

Use builtin cd -- "$target" in templates/fish.fish, bypassing any user cd override (verified: in fish cd is a regular function that builtin cd skips). The -- is retained for consistency with the bash/zsh wrappers; the target is an absolute path written by the binary, so it can never be flag-injected.

Testing

• Added test_fish_init_uses_builtin_cd in src/shell/mod.rs, which fails on the old template (cd -- "$target") and passes after the fix.
• Updated the fish init snapshots (src/shell/snapshots/...init_fish.snap, tests/snapshots/...init_fish.snap).
• Verified fish 3.7.0 behavior directly: cd is a function, builtin cd and builtin cd -- both succeed, and builtin cd bypasses a user-defined cd function (the simulated override was not invoked).
• cargo test --test integration test_docs_are_in_sync passes.

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Closes #3159 — automated triage