Bump command-signatures to pick up git worktree name autocomplete

[warp-agent-staging]

Summary

Bumps the pinned warp-command-signatures dependency so the client ships the git worktree name autocomplete added upstream. None of the upstream work reaches users until the client picks up the new revision.

• Cargo.toml: warp-command-signatures rev 4990fa1d4f5a9c74251c7330484a7ab6f3d38ac1 → a520a43213d3b2eae608b14e3b9b3a11ac4d41aa
• Cargo.lock: the warp-command-signatures and warp-completion-metadata source-rev entries updated to match (scoped to only these two packages — no unrelated dependency churn)

The target revision is the current main HEAD of warpdotdev/command-signatures and corresponds to warpdotdev/command-signatures#302 ("Add git worktree name autocomplete suggestions"), which closes #11542. It adds a worktrees generator sourced from git worktree list --porcelain (main worktree excluded) wired into the git worktree lock/move/remove/unlock args in json/git.json.

Scope verification

• The currently pinned rev 4990fa1 is a strict ancestor of the target a520a43, so this does not regress the previously picked-up pkill completion spec.
• The diff between the two revs is exactly command-signatures#302 — a single commit touching only command-signatures/json/git.json and command-signatures/src/generators/git.rs. No unrelated or breaking upstream changes are dragged in.
• The client consumes command-signatures purely as a compile-time embedded dependency (rust-embed, via signature_by_name / dynamic_command_signature_data()); nothing is vendored, cached, or code-generated in this repo from the specs, and no client test fixture/snapshot asserts over the spec set — so no regeneration step is required for this bump.

Verification

Testing-exempt category: dependency/version-bump (per factory-verification). No new regression test is added — a test here could only assert the pinned rev string, not detect a logic defect; the new behavior lives and is tested in the upstream command-signatures repo (validated by its own CI on #302).

Deterministic checks run locally (writable CARGO_HOME/CARGO_TARGET_DIR):

• cargo check --locked -p warp_completer — passes; --locked confirms the scoped Cargo.lock is self-consistent and warp_completer compiles against the new rev.
• cargo fmt --check -p warp_completer — clean.
• cargo clippy --locked -p warp_completer --all-targets — clean, no warnings.
• cargo nextest run -p warp_completer — 138 passed. The 25 failures are a pre-existing environment issue unrelated to this change: an identical warp_features panic (crates/warp_features/src/lib.rs:1052:13, "Tried to check FeatureFlag::CloudEnvironments before feature flags were initialized"). The same tests fail identically on the unmodified base commit, and none of the failures reference command-signatures/worktree/git.json.

Not user-facing to author in this repo: this is a pin bump with no client-side behavior authored here, so no computer-use visual proof is expected/attached.

CHANGELOG-IMPROVEMENT: Add autocomplete suggestions for git worktree names in git worktree lock/move/remove/unlock
Originating thread: https://warpdev.slack.com/archives/C0BDQDW8V5E/p1785969286177199

This PR was generated with Oz.