feat(events): give a session an event envelope carrying run lifecycle

[justintime4tea]

Implements #778, a sub-issue of #578. Two commits on nightly, now that #710 and #719 have merged.

AgentEvent says what an agent did and nothing about the run it belongs to, so every seam reconstructs a run's story from its transport. This gives the session an envelope that carries both, one ordered stream per session.

feat(events): give a session an event envelope carrying run lifecycle

aura_events::run adds SessionEvent: the session id, the run id when the event belongs to a run, a sequence number dense per session across its runs, a timestamp, and either an AgentEvent or a LifecycleEvent (started, observer attached and detached, claims exhausted, liveness decided, parked, finished, cancelled, failed). No producer emits it yet; #779 is the first.

The items #778 lists to settle before a producer exists:

• Session-keyed. session_id is required, run_id optional, seq dense per session. ends_run() says whether an event ends its run; the stream goes on.
• RunId is a UUID, v7 when minted with RunId::mint(). aura-events takes uuid for it (the CLI already depends on the same workspace crate).
• Adjacent tagging on the payload, kind beside event, so no event field can collide with the envelope's tag. Lifecycle events stay internally tagged by type; the roundtrip test over every variant catches a field named after a tag. The module doc records that the family needs a self-describing format.
• RunCancelReason moves here from aura::hooks (re-exported there), is #[non_exhaustive], and gains Unclaimed and Shutdown.
• ObserverDetached carries a DetachCause (Released, Expired, Displaced).
• Started carries the run's Liveness, read as cancel-at-once when an older payload omits it.

refactor(agents): name a run by its RunId

RunContext::id becomes the RunId, so the envelope's id and the task-local id are one value.

• The chat, A2A, and Slack handlers mint a RunId and use its string form as the request id, so approvals, their sweep, MCP cancellation, and the A2A cancel map keep one value per run.
• begin_run, AgentRuntimeConfig, and RigBuilder's build methods take a RunId; an agent built without one mints its own. The orchestration factory streams under the run id it was built for, as Agent already did.
• Orchestration's RunId is a re-export of the shared type. Orchestration persistence still mints its own value for checkpoints and the park owner key; adopting the run's id there waits for [FEATURE]: A runtime that owns sessions and their runs #780, which owns resume.

Visible change: request ids are hyphenated UUIDs rather than req_<hex>, a2a_<task_id>, and slack_<channel>_<ts>. The A2A executor and Slack runner log the run id beside the task or message it serves, at debug.

Verification

• cargo +nightly fmt --check, cargo clippy --workspace --all-targets --all-features -- -D warnings, and cargo test --workspace (2,560 passed) on nightly at 513f1ec1.
• make check-cli-http-only fails on nightly independently of this change; that is [BUG]: HTTP-only aura-cli build fails clippy with unused args in resolve_env_config #721.

Fixes: GH-778
Ref: GH-578

[greptile-apps]

Confidence Score: 5/5

[High risk] Refactors run identification from strings to structured types.

No new blocking issue was identified, and the PR appears safe to merge on this review.

Summary

The PR adds a session event envelope and shared UUID run identifier, then threads that identifier through the chat, A2A, Slack, agent, and orchestration paths. Since the previous review, it has revised lifecycle comments and added a note about orchestration persistence retaining a separate identifier.

_{Reviews (4) · Last reviewed commit: "refactor(agents): name a run by its RunI..." · Reviewed by Greptile}

[justintime4tea]

Superseded by #794. Renaming this PR's branch to justingross/GH-778-run-events-carry-lifecycle-and-activity closed it; the commits are unchanged.