fix(daemon): scope pairwise a2a transcript reads to the session's own agent - #969

What

Fixes #967. Every postless toAgent child of one caller shares the synthetic a2a:<caller> channel + the caller's thread, so all of a caller's pairwise sessions land in one physical transcript thread — and the §8.5 catch-up and turn-context refresh, built for genuinely shared conversations, replayed siblings' private deliveries (role assignments, canaries included) and reports into each child's prompts. Measured in the webchat Werewolf real run (#941): a player's night prompt carried the referee's role deliveries to other players.

The niche fix

The sessions are pairwise; the reads now are too — the coordinate scheme, session keys, wire, and every genuinely shared conversation are untouched:

• isSyntheticA2aChannel (cp-collab-routes, beside the coordinate minting; the transcript channel key is prefix-preserving so it works on both raw and keyed forms);
• the store gains transcriptSinceForAgent / transcriptSinceRevisionForAgent — the same sender/recipient/transcript_recipient delivery scope the console session views already use, factored into one shared AGENT_DELIVERY_SCOPE_SQL (the three existing inline copies now reuse it, byte-identical SQL);
• the three context seams route through the scoped reads exactly when the session sits on a synthetic a2a channel: the §8.5 catch-up (session-manager), the thread-context refresh (scopeReadsToAgent), and localInvalidatingEvents.

Every a2a writer already stamps recipient (trigger append, observed-inbound, coalesce append), so a pair's own rows are all still visible to it.

Tests

• New packages/daemon/test/a2a-transcript-privacy.test.ts — two children of one caller, two rounds of private deliveries + replies: each child sees its own canary, never the sibling's delivery or reply, across every prompt of every turn. Red without the fix (the sibling's canary appears verbatim in the child's second-turn catch-up), green with it.
• Extended the scripted webchat Werewolf gate: the runner now audits every player prompt (conversation AND pairwise sessions) for role canaries the player's role does not hold — canaryCrossVisibility pinned 0 in both CI games. This is the live-topology regression probe: role secrecy in the single-conversation game now actually holds.
• Unmodified: daemon webchat + feat(daemon): let webchat agent posts continue a multi-agent conversation (#549 parity) #906 continuation 47, session-manager 95, daemon-transcript, eval:collab:contracts 119, eval:parity 23, full typecheck + lint.

Expectations were only extended (the two new pins); nothing weakened.

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