[2/2] feat(eve): observe channel deliveries as session history

[Lukavyi]

Stacked on #3020. Review the last commit only (feat(eve): observe channel deliveries as session history); the first two commits are #3020 and will disappear on rebase once it lands.

Summary

A group bot only ever sees the messages that address it. Even with one session per chat (#3020), it cannot follow the conversation that happens between mentions, so "@bot, what do you think about what U1 just said?" has nothing to work with. Slack has threadContext for this; Telegram had no equivalent, and the dispatch gate drops unaddressed messages before they reach a session (#874, second half).

This adds observe: true to channel deliveries, a channel-agnostic primitive:

• An observed delivery appends its message and context to the session as history without running a turn. eve buffers it on the session, parked or mid-turn, and folds the buffered messages in arrival order into the next delivery that does run a turn, so the model sees them as part of that turn's input (this rides on the coalescing fixed in coalesceDeliverPayloads silently drops earlier messages in a batched delivery (last-key-wins for message/context) #897).
• It never steers an active turn (turnPolicy is not applied) and never starts a session: with no owner for the address it throws RuntimeNoActiveSessionError, which the Telegram channel logs at debug and drops.
• The observed backlog is capped at 256 payloads, oldest dropped first. Task deliveries settle on their own and leave observed history waiting for a channel turn, so a child result never carries chat history it did not ask for.
• Telegram exposes it as observe on the onMessage result. isTelegramBotMentioned is now exported so a custom onMessage can gate on it; the docs carry the four-line example. Telegram only delivers unaddressed group messages with privacy mode off or the bot as admin, noted in the docs rather than enforced.

SendPayload gains a field, so the channel extension contract moves to epoch 18 (structurally backward compatible, epoch 17 retained).

We have run this shape as a patchedDependencies patch in production on two group bots since July; the acceptance scenario from #874 (untagged question, later a mention, answer with that context) passes there.

Related to #874, #897, #223 (Slack: the same observe gate would let a Slack onMessage follow a thread without mentions).

Validation

pnpm --filter eve exec vitest run --config vitest.unit.config.ts src/execution src/channel src/public/channels/telegram   # 124 files, 1109 passed
pnpm --filter eve exec tsc -p tsconfig.json --noEmit
pnpm run guard:invariants
pnpm run docs:check
pnpm update:extension-contracts --update channel

New coverage: coalesceDeliverPayloads keeps observe only when every payload was observed; bufferObservedDelivery buffers observe-only deliveries, leaves addressed ones to the caller, and drops the oldest past the cap; a parked session stays parked on observed deliveries and coalesces them ahead of the next addressed one; buffered observed deliveries alone do not wake it; a task delivery runs without them; the turn receiver buffers observed deliveries without steering or forwarding; the Telegram channel delivers an observe result as history with the <telegram_context> block and a mention without it.

Checklist

• This change was requested or approved by a maintainer (asked in Telegram: group mentions start a new session per bot message; key groups chat-wide like private chats #874, awaiting an answer)
• I ran the relevant checks from CONTRIBUTING.md
• I added tests and documentation where relevant
• I added a changeset if this touches the published eve package
• DCO sign-off passes for every commit (git commit --signoff)

🤖 Generated with Claude Code

https://claude.ai/code/session_015P6RBuagWTMzBmJLCiYKW9

[vercel]

@Lukavyi is attempting to deploy a commit to the Vercel Team on Vercel.

A member of the Team first needs to authorize it.