PR 3: remove TipTap — legacy editor deletion + breaking cleanups

[rpvilo]

Final step of the editor migration (#32 seam, #33 engine). Do not start until the end condition holds.

End condition

• Manual checklist on feat(composer): hand-rolled contenteditable editor engine #33 passed in Safari + Chrome (incl. IME items)
• ~2 weeks of daily use with the new editor as default, no flips back to TextareaLegacy

Deletions

• packages/chat/src/composer/textarea-legacy.tsx (+ TextareaLegacy from the compound and ComposerTextareaLegacyProps export)
• packages/chat/src/composer/document.ts (TipTap conversions + adapter; keep nothing — serializeSegments replaced serializeDocument)
• packages/chat/src/composer/mention-chip.tsx
• packages/chat/src/composer/prefix-plugin.ts (pure kernel already lives in prefix-detection.ts)
• packages/chat/tests/composer-document.test.ts (its serialize fixtures are already mirrored in composer-segments.test.ts)
• Deps from packages/chat/package.json: @tiptap/core, @tiptap/extension-document, @tiptap/extension-paragraph, @tiptap/extension-text, @tiptap/pm, @tiptap/react, prosemirror-model, prosemirror-transform, prosemirror-view
• Dead index exports: commandListPluginKey, createMentionChipExtension, MentionChipOptions, applySnapshotToEditor/snapshotFromEditor/serializeEditorContent, createTiptapRegisteredEditor

Breaking cleanups (batched here while pre-1.0)

• Rename CommandListPluginState → engine-neutral (e.g. ActiveTokenState) — no plugin exists anymore; update prefix-detection.ts, trigger-tracker.ts, index exports, tests
• Rename ComposerSnapshot.__pmDoc → neutral field name (payload is already engine-neutral paragraph JSON; snapshots are cross-engine compatible so no migration needed)
• ComposerState.textarea slice goes data-only ({ hasContent }) — imperative methods live on store.controller / useComposerController only
• Rename the TriggerRule value "after-whitespace" → "word-boundary" (it also fires at line start — the current name is factually wrong). Touches types.ts, prefix-detection.ts docs, demo command configs, docs tables. Decided 2026-07-10: keep kind: "insert" | "execute" and the trigger field name as-is (minimal rename).
• Docs: drop the TextareaLegacy mention from COMPOSER.md; final sweep for TipTap references

Verification

• bun test, tsc --noEmit, biome — green
• rg -i "tiptap|prosemirror" packages/chat → zero hits
• Package builds; bundle no longer contains PM (size drop is the payoff)
• Demo smoke test after dep removal