feat: per-page command registration API for command palette

[mikewheeleer]

Summary

Context: as noted on #924/#968, this repo has a large amount of previously-deleted UI infrastructure from a prior incident. Neither CommandPalette nor CommandPaletteProvider exist currently -- both were among the deleted files (root layout.tsx already references <CommandPaletteProvider /> but doesn't compile, for unrelated reasons, same incident). Per maintainer guidance, building both fresh rather than resurrecting the old implementation, with the registration API this issue actually asks for from day one rather than a hard-coded list.

What this adds

• CommandPaletteProvider -- holds open/close state, a small static navigation command set, and a registry of dynamically-registered commands. Merges both into the commands array exposed via context (deduped by id, dynamic registrations win on collision). Global Cmd/Ctrl+K toggles the palette.
• useRegisterCommands(commands) -- registers on mount, unregisters on unmount.
• CommandPalette -- the searchable list UI, built on the existing Dialog primitive (focus trap, Escape-to-close, inert background). Keyboard-navigable via the aria-activedescendant pattern (Up/Down/Enter on the input, no need to tab into the list); mouse click is a secondary affordance.
• docs/COMMAND_REGISTRATION.md -- API reference, usage example, accessibility notes.

A real bug found and fixed while building this

My first pass made useRegisterCommands's effect depend on the commands array reference (the "obvious" design). Writing this PR's own tests, I hit an actual out-of-memory crash: a component that both calls useRegisterCommands([command]) (a fresh array literal each render) and reads commands back from useCommandPalette() enters an infinite loop -- register → state update → re-render → new array reference → re-register → ... This is an easy trap for real page code to fall into (e.g. a page showing "N commands available" alongside registering its own).

Fixed by re-registering only when the set of ids changes (a stable primitive key derived from sorted ids), not on the array reference. A non-memoized array is now safe to pass every render. Documented the trade-off (in-place content mutation on an unchanged id won't take effect -- give the command a new id, or memoize with useMemo, if that's needed) in the hook's docstring and in the doc file.

Testing

• useRegisterCommands.test.tsx -- 8 tests: static defaults present, register-on-mount merges with static set, unregister-on-unmount, id-based dedupe (via direct registerCommands calls), multiple simultaneous registrants, manual registerCommands returns a scoped unregister fn, open/close/toggle, throws outside a provider.
• CommandPalette.test.tsx -- 6 tests: closed renders nothing, lists static commands when open, filters by query, "no matches" state, Enter runs the highlighted command and closes, click runs a command.
• All 14 new tests pass; confirmed via git stash A/B comparison that this diff adds zero new build errors and zero new test failures elsewhere (baseline: 22 failed test files / 37 failed tests / 488 passed, unchanged except my +14 passing).

Closes #918

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