feat(web): @ autocomplete to cite other sessions by title (v1 plain-text)

[heavygee]

Summary

Composer @ offers fuzzy autocomplete over session titles, inserting the same citation grammar as Copy reference (See session "…" (/sessions/<id>) for context).

Scope (v1 only)

• @ autocomplete over fleet session list (all agent flavors)
• Plain-text expansion (not rich chips / not attachment tray)
• Codex file @ mentions still work (session hits listed first)
• /sessions/<id> autolinks in chat markdown (in-app navigate)
• Reuses existing slash-command autocomplete stack

Out of scope (see follow-up)

• Inline @people-style tokens in a rich/segmented composer → v2 issue

Acceptance

• Typing @ + partial title surfaces the right session among hundreds
• Insert matches Copy reference grammar
• Codex file @ unbroken
• Hard-reload dogfood on soup OK

[github-actions]

Thanks — I verified the current state in code.

@ in the composer is already wired only to Codex file mentions for Codex sessions (web/src/router.tsx:955-975). /$ are handled by the existing slash-command and skill suggestion sources, so there is not yet a session-title autocomplete source in the composer pipeline.

The data you need is already available on the web side: GET /sessions returns the full namespace session list, useSessions() consumes it, and session titles are derived from getSessionTitle() / searchable fields like metadata.name and metadata.summary.text (hub/src/web/routes/sessions.ts:65-100, web/src/hooks/queries/useSessions.ts:6-28, web/src/lib/sessionTitle.ts:10-21, web/src/components/SessionList.tsx:498-515).

Copy reference already emits the citation text you want via buildSessionReferenceText() (web/src/lib/sessionReference.ts:11-18, web/src/components/SessionActionMenu.tsx:197-205). Relative /sessions/<id> links are allowed through the markdown anchor handler, so they can navigate in-app (web/src/components/assistant-ui/markdown-text.tsx:523-583).

Relevant code:

• web/src/router.tsx:955-975 - current @ / $ / / autocomplete dispatch
• hub/src/web/routes/sessions.ts:65-100 - sessions API returns the whole namespace
• web/src/lib/sessionTitle.ts:10-21 - session title derivation
• web/src/components/SessionActionMenu.tsx:197-205 - Copy reference path
• web/src/components/assistant-ui/markdown-text.tsx:523-583 - relative /sessions/... links pass through for navigation

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HAPI Bot

[heavygee]

Split for #1213

• v1 (shipping): @ autocomplete + plain-text expansion to the existing Copy-reference grammar (See session "…" (/sessions/<id>) for context). /sessions/<id> autolinks in the transcript. Same slash-command autocomplete stack.
• Not v1: attachment-style composer chips — dogfooded and reverted. They cannot express positional “this clause → session A / that clause → session B.”
• v2 (follow-up): segmented rich composer (inline session tokens at caret). Track separately when we kick that off.

[heavygee]

Split: this issue is v1 only (plain-text expansion). Rich composer follow-up: #1216

[heavygee]

v2 follow-up tracked in #1215 (rich/segmented composer).

[heavygee]

Correction: v2 tracking is #1216 (not #1215 — that was a duplicate and is closed).