Apply session _meta once per adapter update and keep it transactional

[ulugbekna]

Follow-up to #327629 ("quick chats: fix: show within Chats section instead of random UUID dirs"), now merged. Rebased onto main; this PR is a single commit against main.

Scope, stated plainly: the behavioural fix for the quick-chat grouping bug shipped in #327629. This one is an optimization, its prerequisite, and two hygiene items. Sizing it that way up front so it can be triaged accordingly — or trimmed, see "If you want to trim" below.

#327629 made _meta arrive on every listSessions refresh. _refreshSessions() fires on each ChatTurnComplete and calls update() for every listed session (the cache caps at 100), so anything redundant in AgentHostSessionAdapter.update() is now per-session, per-turn work.

---

1. Apply _meta once per update — optimization

update() applied _meta twice: once inline, then again through the trailing setMeta. With promotion now living inside setMeta, the duplicate also re-ran _promoteToQuickChatIfWorkspaceless and _computeWorkspace() — the latter allocating a workspace object and doing a configurationService.getValue('git.branchProtection', { resource }) lookup — plus a second agentHostSessionWorkspaceKey() (an 11-field string join).

Applied once now, up front, where the promotion still precedes the workspace rebuild. Halves that work. Real but modest — not something you'd notice in a profile.

2. setMeta joins the caller's transaction — prerequisite + hardening, not a live bug fix

Being straight about this one: on main today there is no tearing. setMeta is the last write in both transactional callers (update() and _handleSessionSummaryChanged), so its nested transaction() flushes after everything else is applied.

It's needed because of change 1. transaction() never joins an enclosing one — it constructs a TransactionImpl and finish()es it — so collapsing to a single application at the top of update() is exactly what opens the window: observers of _meta / isQuickChat / workspace would be notified before isArchived, isRead, changes and activity land, handing them a torn snapshot.

setMeta now takes an optional ITransaction and uses subtransaction; update() and _handleSessionSummaryChanged thread theirs through. The two standalone callers (_applySessionMetaFromState, _handleSessionMetaChanged) pass nothing and keep their own transaction, so their behaviour is unchanged.

Beyond unblocking change 1, this removes an ordering landmine: today the invariant "setMeta must be the last write" is implicit and unenforced, so adding any field write after it silently reintroduces torn notifications.

3. Doc key correction + negative wire test — the part I'd argue actually matters

• copilot.workspaceless appears nowhere in the code. The marker is persisted as agentHost.workspaceless (AH_META_WORKSPACELESS_DB_KEY), written centrally by AgentService; the agents only read it. The wrong key was named in AGENT_HOST_SESSIONS_PROVIDER.md and in .github/skills/sessions/SKILL.md — the latter is read by agents, so it keeps propagating.
• Pins the absent-_meta case on the wire. The item is built field by field and satisfies SessionSummary catches a dropped optional in neither direction — which is exactly how this bug shipped twice.

---

If you want to trim

The honest minimum is section 3. Sections 1 and 2 travel together (2 exists to make 1 safe) and can be dropped without affecting anything user-visible.

Testing

• a refresh publishes _meta and summary fields as one atomic update — asserts an autorun observing workspace + isArchived sees exactly two snapshots across a refresh that moves both. Re-verified after the rebase that it fails with a torn { feature, false } middle snapshot when subtransaction is reverted to transaction.
• listSessions omits _meta when the agent provides none at the protocol layer.
• npm run typecheck-client clean, eslint clean, 532 tests passing across the local/remote provider, protocol server/client, agent service and sessions-list suites.

Review provenance

#327629 and an independent branch of mine were produced by two separate sessions that reached the same root-cause diagnosis and the same wire fix, then diverged on how to heal already-mis-classified sessions. Rather than merge them, I kept #327629's approach and reduced mine to the deltas above.

• Plan review (3 frontier models, on my original branch) unanimously rejected making the adapter's session-kind mutable — the concerns were an open session's workspace flipping to undefined mid-session and supportsMultipleChats collapsing chat tabs — and recommended dropping the persisted cache via a storage-key bump instead. Keep workspace-less quick chats out of bogus workspace groups #327629 shipped the mutable-kind approach anyway, with a one-way promotion guard, and it is the better call: it heals in place with no cold-start cost and covers the remote provider for free. My cache-key bump and _metaByRawId guard were dropped as redundant.
• Implementation review (3 frontier models). One caught the nested-transaction hazard described in section 2 in my original branch, where it was a live regression I had introduced by reordering; that finding is what section 2 carries forward as hardening here. Another verified the update() rewrite is equivalent to the original in didChange propagation and notification timing, and that the preserved metadata is contained to _persistCache. A third flagged the remote provider's cache key and withdrew after the offline-list tradeoff was explained.

[Copilot]

Pull request overview

Makes agent-host session metadata updates transactional and avoids duplicate _meta application.

Changes:

• Threads transactions through setMeta.
• Adds atomic refresh and protocol metadata tests.
• Corrects workspace-less persistence key documentation.

Show a summary per file

File	Description
baseAgentHostSessionsProvider.ts	Updates transactional metadata handling.
localAgentHostSessionsProvider.test.ts	Tests atomic refresh publication.
protocolServerHandler.test.ts	Tests absent _meta serialization.
AGENT_HOST_SESSIONS_PROVIDER.md	Corrects the persistence key.
.github/skills/sessions/SKILL.md	Corrects session guidance.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 2
• Review effort level: Medium

The base branch was changed.