feat(router): seed thread parent into fresh per-thread sessions

[apparentsoft]

Summary

Adds an optional fetchThreadParent hook on ChannelAdapter, plus router wiring that calls it once per inbound event (memoized) when a fresh per-thread session is being created. The fetched parent message is stored on the inbound's replyTo field so the container's formatter renders it as <quoted_message> in the agent's view.

Framework only. No adapter implements fetchThreadParent in this PR — without an implementation the behavior is byte-identical to today. The Slack implementation lands in a follow-up PR against the channels branch that depends on this merging first.

Motivation

Today, when a user starts a Slack/Discord thread by replying to some earlier message, and that thread's first reply triggers a new per-thread session, the agent wakes up with no view of what was being replied to. It sees only the reply text in isolation, with no context for why the user is saying it.

Seeding the originating top-level message into replyTo gives the agent the missing context on its very first turn, without changing anything for adapters that don't opt in.

What changed

• src/channels/adapter.ts — adds optional fetchThreadParent?(platformId, threadId): Promise<{id, sender, text} | null> to the ChannelAdapter interface, with doc on fail-open semantics.
• src/router.ts — two pieces:
  1. Memoized fetcher at the top of routeInbound: lazy, cached per event, fail-open (a throw or null collapses to "no parent"). Avoids redundant platform API calls when multiple agents wired to the same channel each create a new session.
  2. Seeding block at the top of deliverToAgent: fires only when created && event.threadId && adapterSupportsThreads. Parses the inbound content as JSON; if it's not JSON or already has replyTo, skips. Otherwise sets replyTo = {id, sender, text} from the fetched parent, but only when the parent's id differs from the inbound's id (so single-message threads don't get a self-quote).

Safety properties

Condition	Behavior
Adapter doesn't implement fetchThreadParent	No-op
Adapter throws	Logged warn, treated as null
Returns null	No seeding
Inbound is not JSON	Skipped (legacy/native adapters)
Inbound already has replyTo	Not overwritten
Parent id === inbound id	Not seeded (single-message thread)
Not the first message of the session (created=false)	Not seeded
Not a threaded adapter	Not seeded
Inbound has no threadId (DM)	Not seeded

Test plan for review

• pnpm run build clean
• pnpm test — 332/332 pass on this branch
• Reviewer: visual diff confirms deliverToAgent signature change is consistent at both call sites in routeInbound (engaged and accumulate paths both pass getThreadParent).
• Reviewer: confirm no adapter currently in trunk implements fetchThreadParent (it doesn't — channels live on the channels branch only).

Follow-up PR

Slack implementation (src/channels/slack-thread-parent.ts + wiring in src/channels/slack.ts) against the channels branch. I'll open it as soon as this lands so reviewers can see the actual user-facing behavior end-to-end.

Compatibility

No breaking changes. No schema/DB changes. No env changes. Interface addition is optional.