fix(ai): dispatch skills registered in the slash command registry

[BinaryMuse]

Since #3525, resolve_skill_name required a skill's name to be absent from the slash command registry (so built-ins take precedence), but every skill is registered there for autocomplete; every skill invocation fell through to "Unknown command".

This PR decides precedence with an is_builtin flag on SlashCommand instead of registry membership, and extracts the predicate into a testable function with regression coverage.

[greptile-apps]

Greptile Summary

Skills registered in the slash command registry for autocomplete were incorrectly blocked from dispatching because the old predicate treated registry membership as exclusionary. This PR fixes that by adding an is_builtin flag to SlashCommand and keying precedence on that flag instead.

• SlashCommand::builtin() constructor and contains_builtin() method replace the old contains() check, correctly allowing non-builtin (skill-backed) commands to dispatch.
• Three unit tests cover the regression, the builtin-wins case, and the unregistered-name case.

Confidence Score: 5/5

Straightforward targeted fix with good regression coverage; no risk to unrelated paths.

The change is narrow and well-contained: one new struct field, one constructor, one renamed method, and three tests that directly validate the corrected behavior. All existing builtins are updated to use the new constructor.

No files require special attention.

Important Files Changed

Filename	Overview
crates/atuin-ai/src/tui/slash.rs	Adds is_builtin flag to SlashCommand, a builtin() constructor, and replaces contains() with contains_builtin(). All existing built-in commands updated correctly.
crates/atuin-ai/src/driver.rs	Extracts is_skill_command predicate that uses contains_builtin instead of contains, fixing skill dispatch. Three regression tests cover the key cases.

_{Reviews (1): Last reviewed commit: "fix(ai): dispatch skills registered in t..." | Re-trigger Greptile}