feat(mentions): Slack-style inline @-mentions in chat + agent composers, avatar circle fix

[th3-br41n]

What

User-reported (dogfood): in Chat, mentioning a person dropped a chip above the composer and the sent message carried a boxed attachment chip instead of the mention rendering inline in the text; user avatars rendered as a square inside a circle. Expected Slack behavior, identical in Chat and Agent.

Editor / SDK

• MentionComposerPlugin gains an insertNode commit mode: committing a typeahead row replaces the @token with a real inline MentionNode chip + trailing space. The tokenText (comments) and excise modes remain; onSelect is now optional.
• CompactEditorHandle.setText (discrete update) — lets the Agent seed intent-driven drafts.
• The Lexical-free renderEditorState now renders mention nodes as inline chips (same classes as MentionNode.decorate()) instead of the ⟦mention⟧ fallback.
• extractNoteReferences carries the mention chip's denormalised label (clamped to 256).
• New composer-context helpers, shared by both apps:
  • inlineMentionRefs(richBody) — the mention chips in a serialized rich body;
  • withMentionAttachments(richBody, atts) — lifts inline mentions into durable Person/Entity attachments (the mention-notifier and agent grounding keep working — the mention still travels on the wire, it just doesn't render as a chip);
  • visibleAttachments(msg) — hides a chip whose mention already shows inline; legacy messages keep their chip; media always shows.

Chat

• Composer registers MentionNode + insertNode — no rail chip on mention; send lifts inline mentions into wire attachments; transcript hides mention-duplicated Person chips.
• Avatar fix: image avatars now fill the circular host (they were 28px rounded squares centred in a 34px ring — the "square inside a circle"); glyph avatars stay centred; members-panel avatars sized to their 26px row.

Agent

• The <textarea> composer is migrated to the same CompactEditor + MentionComposerPlugin surface (hand-rolled-control baseline tightened). User turns persist richBody and render rich; mention-derived attachments ground the turn without rendering as chips.

Verification

• Real-shell dogfood spec 377-inline-mentions (harness repo): all PASS — inline chip in both composers, zero rail chips, sent message renders the mention inline with zero attachment boxes.
• Full suite: 1424 files / 14179 tests green (includes new plugin commit-mode tests, preview mention test, walker label tests, lift/hide helper tests).
• bun run verify + lint (css-token, control-faces, reactivity ratchets) clean.

Security

No new capability surface, IPC method, or dependency. The mention walker runs over peer-writable richBody — it reuses the existing depth-capped, fail-soft extractNoteReferences, and the lifted label is length-clamped.

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