Add Project Update + Project description as agent surfaces (Workstream A)

[alicejt]

What this does

Cyrus today only listens for Issue-level Linear events, so the Project surface — the Updates feed and the description body — is invisible to agents. This adds two capabilities:

A1 — Project Updates as a conversation surface

An @<agent> mention inside a Linear Project Update spawns a conversational agent session bound to that project, and the agent replies as a follow-up Project Update. It reuses ChatSessionHandler (the Slack-chat engine), so the session runs in a plain workspace directory — no git worktree — because the surface is discussion, not code. The project is the thread: follow-up Updates on the same project resume the same session.

A2 — Project description as standing context

On a Project webhook whose description changed, the description is cached; when an agent later picks up any issue under that project, the cached description is injected into its system prompt as long-running context. Caching is opt-in via CYRUS_PROJECT_CACHE_URL / CYRUS_PROJECT_CACHE_TOKEN env vars — when unset it's a no-op, so this is inert for installs that don't want it.

Routing — self-filtering, not central routing

ProjectUpdate and Project webhooks are workspace-wide: every installation subscribed to them receives every event (unlike Issue/AgentSession webhooks, which are agent-targeted). So instead of a central router picking an agent, each instance self-filters — it resolves its own Linear identity (from linearAgentId/linearAgentName config, else the viewer query at startup) and acts only when it is the agent @-mentioned. It never replies to its own Updates (loop prevention).

Changes by package

• core — ProjectUpdateWebhook / ProjectWebhook types + isProjectUpdateWebhook / isProjectWebhook / isProjectDescriptionUpdateWebhook guards; optional linearAgentId / linearAgentName config fields.
• linear-event-transport — fetchProject + createProjectUpdate helpers on LinearIssueTrackerService.
• edge-worker — handleProjectUpdateWebhook / handleProjectWebhook wired into handleWebhook; LinearProjectChatAdapter (a ChatPlatformAdapter for Project Update conversations); ProjectDescriptionCache client; project-mentions.ts (mention parsing + the self-filter gate); self-identity resolution at startup; <project_context> injection into new-session system prompts.
• Unit tests for the type guards, the mention routing gate, the adapter, and the cache client.

Subscription / config (operator-facing)

To enable, subscribe the Linear app to the ProjectUpdate and Project webhook events. handleWebhook already ignores unknown event types gracefully, so subscribing before/without this change is harmless. A2's cache is off unless the two env vars are set.

Testing

• New unit tests pass; full core + edge-worker suites show no new failures (pre-existing screenshot-upload-hooks / runner-selection failures on main are unrelated).
• pnpm build + pnpm typecheck clean across the monorepo.
• Draft until the end-to-end test plan (direct mention → reply, no-mention quiet, description-as-context, cache invalidation) is exercised against a live workspace.

🤖 Generated with Claude Code

[alicejt]

Validated end-to-end against a live Linear workspace with an isolated test instance:

• Spike — confirmed Linear encodes an agent @-mention in a Project Update body as plain @<name> text; the self-filter matcher catches it.
• A1 — @<agent> in a Project Update → worktree-free chat session → reply posted as a follow-up Project Update. Loop prevention confirmed (the agent's own reply Update is ignored).
• A1 no-mention — a plain Project Update with no mention is received and ignored, no session spawned.
• A2 — editing a project's description fires a Project webhook → description cached; a new issue under that project picks it up and the agent's reply reflects it (verified the agent used the description with zero tool calls — it came purely from the injected <project_context>). Re-editing the description invalidates the cache; the next issue picks up the new value.

All four test-plan items pass. Marking ready for review.