Use composable autocomplete provider instead of replacing the editor

[LinYS77]

Summary

This PR changes pi-fff to provide its FFF-backed @ file mentions through Pi's composable autocomplete provider API instead of replacing the
editor component.

The current implementation installs a custom editor with ctx.ui.setEditorComponent(...) only to customize autocomplete behavior. That gives
pi-fff ownership of the whole editor surface, even though the extension only needs to contribute file mention suggestions.

Using ctx.ui.addAutocompleteProvider(...) keeps the same @ mention functionality while allowing pi-fff to compose cleanly with other editor
extensions.

Changes

• Remove the custom FffEditor wrapper.
• Stop calling ctx.ui.setEditorComponent(...).
• Register FFF @ mentions with ctx.ui.addAutocompleteProvider(...).
• Delegate non-FFF autocomplete behavior to the existing provider.
• Keep mode/tool behavior unchanged.

Testing

• cd packages/pi-fff && bun test test/
• cd packages/fff-node && npm run build
• cd packages/pi-fff && bun run typecheck
• git diff --check

[dmtrKovalenko]

@SamuelLHuber do you have any objections?

[dmtrKovalenko]

@gustav-fff review this PR

[SamuelLHuber]

looks good reading it. will test the branch now

[SamuelLHuber]

good one. tested and verified locally on my machine too

[gustav-fff]

[triage-bot] DIRECTED: LGTM.

Composition via addAutocompleteProvider((current) => ...) is correct. Mention path tries first, falls through to current on null/abort/throw. shouldEnableMentions() evaluated per-call so /fff-mode toggles take effect without re-registration — the removed applyEditorMode re-apply on mode change is intentional and safe.

Notes (non-blocking):

• applyCompletion always delegates to current. Works because item.value already encodes @"path with space"/@path, so prefix-length replace produces the right result. If a future base provider gets clever about prefix handling (e.g. unquoting), mention insertion could regress — worth a single inline test that asserts the inserted line for an @-mention.
• catch {} in getSuggestions swallows native errors silently. Acceptable for autocomplete UX, but a pi.notify/debug log on first failure would help diagnose FileFinder startup races (pi-fff: parallel ffgrep calls can hang indefinitely due to unsynchronized ensureFinder singleton #403-class issues).
• shouldTriggerFileCompletion defaults to true when base lacks it — fine for mentions, but could double-fire base file completion alongside @ results in some hosts. Watch in real session.

Tests pass: bun test test/ → 17 pass.

Honk-Honk 🪿