Remove SessionSummary from SessionState

[connor4312]

There are initial debt items around here...

• SessionSummary has model/agent selection, but these can probably just live on the SessionState
• Reducers documented as "pure" stamp summary.modifiedAt from the wall clock (Date.now); TS client lacks the now-seam the other clients have #186
• createdAt/modifiedAt is numeric on SessionSummary but an ISO timestamp on ResourceResolveResult. I think ISO is a preferred representation.
• nit: changesets on the summary is probably too verbose/noisy

...but while we're here, the whole situation of the SessionSummary being embedded on the SessionState is bad. We have two different sets of actions to update the same thing, which has led to confusion in implementation and makes it really hard to fix #186 nicely.

SessionSummary should share some fields with SessionState, but it should be its own thing that exists solely on the root. In that case the host is just in control of updating the modifiedAt at its own discretion and get we get rid of the oddly overlapping actions.

We can also reduce how much of changesets we want to expose on the summary.

Fix timestamps at the same time.

[joshmouch]

Adding consumer-side empirical data that's relevant to how this restructure handles modifiedAt, since it bears directly on #186.

I built a standards-compliant AHP consumer against the published @microsoft/agent-host-protocol@0.2.0 (AhpClient + WebSocketTransport, driving the published sessionReducer), and hit a concrete gap worth flagging here:

The published wire ActionEnvelope has no host-authoritative metadata channel. It is exactly:

// dist/types/common/actions.d.ts
export interface ActionEnvelope {
  channel: string;
  action: StateAction;
  serverSeq: number;
  origin?: ActionOrigin;
  rejectionReason?: string;
}

No meta / modifiedAt / serverTimestamp. Combined with #186 (the reducer stamping summary.modifiedAt = Date.now()), this means a compliant client converges on the host's authoritative modifiedAt only via the snapshot / reconnect-snapshot path — not on the two legs that replay bare actions:

1. Write-ahead optimistic apply — the client replays its own pending actions over confirmedState (docs/guide/reconciliation.md), re-running the reducer → re-stamping modifiedAt from the client's clock. No snapshot in this leg.
2. Reconnect-replay — the ReconnectResult.Replay branch streams missed action envelopes; those carry no timestamp, so the reducer re-derives modifiedAt again.

So #186's now-seam makes modifiedAt deterministic per client, but nothing on the wire makes it converge to the host's value on those replay legs.

Question for this issue: does moving SessionSummary (and modifiedAt) off SessionState to a root, host-owned location intend to make modifiedAt purely host-authoritative such that clients never re-derive it during action replay? If so, #187 likely resolves the convergence gap above outright (and a per-action envelope field would be unnecessary) — which is the outcome I'd prefer.

If #187 ends up deferred, or if a residual per-action authoritative-meta need remains, the smallest additive alternative would be an optional meta?: { modifiedAt?: number } on the envelope (host stamps once; clients overlay verbatim post-reduce, never re-derive; absent ⇒ today's behavior). I wrote that up as a fuller draft with the firsthand evidence and alternatives — happy to share it or open it as a separate issue only if you'd find it useful; I didn't want to file a competing proposal against your preferred direction here.

Mainly flagging the replay-leg data so the restructure can account for it. Thanks!

[connor4312]

Question for this issue: does moving SessionSummary (and modifiedAt) off SessionState to a root, host-owned location intend to make modifiedAt purely host-authoritative such that clients never re-derive it during action replay

The host would explicitly update modifiedAt during the root/sessionSummaryChanged action. No implicit updates. This is already possible since that action just has changes: Partial<SessionSummary>.