Story-2097: Memory Engine ME-3 — the ACP session dropdown

The Agent Chat Panel's session dropdown grows from four verbs to the full lifecycle the Memory Engine defines. Per pramaan-functions docs/notes/rfd-memory-engine.md (RFD-MEM-001) §4.3 and §8, a counsel keeps several parallel conversations per agent and manages each one — new · load · rename · compact · clear · archive/restore · delete — with the auto-resolved default session pinned first and the rest ordered by most-recent activity.

"Return to the thread where we discussed limitation." — the session is the conversation container; the dropdown is how counsel moves between them.

The ME-2 contract seam (why some verbs are wired and some are not)

The functions-side session routes are ME-2 (pramaan-spec #2096). Their OpenAPI snapshot is not yet in this repo's src/gen mirror — the mirror carries only /app/agents/session-context/resolve, not the agent-sessions lifecycle routes. Per the story's own guardrail, pnpm codegen was not run here. So ME-3 ships the dropdown against the existing client-side session cache (agent-session-store.ts, Story-1721), and the verbs whose truth is server-side attach at a named seam:

• Client-owned, live now: new, load/switch, rename, delete, archive/restore — these are list-state the dropdown itself renders, so they run against the store and update instantly.
• Server-owned, seam: compact (fold earlier turns into the summary) and clear (soft-reset) are the ME-2 POST …/compact|clear routes. SessionMenu renders them only when the caller passes onCompact/ onClear; the live Atlas dock omits both today (a commented seam in agent-dock-mount.tsx). The moment the codegen-sync chore lands the ME-2 contract, those callbacks pass the generated mutations — no component change.

The store — the dropdown's list model (ME-3 additions)

• src/lib/agents/agent-session-store.ts — AgentSessionMeta gains four fields the dropdown reads: availability ("live" | "archived"), isDefault (the pinned default_agent_chat, RFD §4.1), lastActiveAt (ordering), and compactedTurns (the display-only fold marker, populated by ME-2 — the client never fabricates a turn count).
• The snapshot is now sorted pinned default first, then lastActiveAt DESC (RFD §8). New verbs archiveSession/restoreSession; the default can never archive or delete (it is the dock's guaranteed home thread), and archiving or deleting the active session re-homes the binding to a live session. switchSession stamps lastActiveAt and refuses an archived target. The persisted envelope bumps to v: 2 so an old client's rows are discarded safely (HR-23).
• use-agent-sessions.ts splits the snapshot into sessions (live) and archivedSessions and exposes archive/restore.

The dropdown — SessionMenu

• src/components/agents/session-menu.tsx — the live list shows a pin on the default row and a condensed N marker on a compacted session; the active session's verbs are Rename · Compact · Clear · Archive · Delete (Archive/Delete disabled for the default). An "Archived sessions (N)" item reveals the restorable archived view inline (RFD §8's archive-management-list doctrine). Clear and Delete each confirm; Archive is one-tap (restorable). Purely presentational — all state lives in the store.

Namespace isolation — Mira vs Atlas

A Mira session and an Atlas session on the same matter are different rows, different dropdowns, different memory scopes (RFD §4.1). This is structural: the session scope key carries the agent_key axis (buildSessionScopeKey), so the two agents' lists can never collide — proven by a store test asserting an Atlas scope and a mira:<matterId> scope stay independent. Atlas's dock drives the dropdown fully today; Mira's dock adopts the same control once ME-2 session-keys her conversation rail (that rail is matter-keyed today — a separate ME-2 seam).

i18n & stories

• New Agents.* keys (sessionDefault, sessionCompacted, compactSession, clearSession, clearSessionConfirm, archiveSession, restoreSession, archivedHeading) added to both messages/en.json and messages/hi.json.
• session-menu.stories.tsx covers the dropdown states: Default (pinned + ordered), SingleDefaultSession (archive/delete disabled), Compacted (the fold marker), FullVerbsAndArchived (compact + clear wired, archived view), and an interactive Live harness.

Verification

• pnpm test — 1024 passed (store lifecycle/ordering/archive/isolation cases added to __tests__/agent-session-store.test.ts).
• pnpm exec tsc --noEmit, pnpm lint, pnpm build — clean.
• Component states verified in Storybook (Storybook-first doctrine); the live "drives all verbs against staging" acceptance is contract-blocked on the ME-2 codegen-sync and named as the seam above.