Poll Open Linear issues for current readiness

[ferueda]

What changed

• generalize the Backlog poller into a small issue poller that preserves revision-based Backlog observations
• add per-cycle readiness checks for Open issues when Spec or Implement routes are composed
• refetch and confirm current Linear state, action labels, and blockers before dispatch
• derive observed states from the same worker route map that enables consumers
• document the event identity and composition contract

Why

A blocked Open issue can become ready when its blocker changes state without changing the blocked issue revision. Revision-only polling therefore cannot reliably wake future Spec or Implement work. Per-cycle readiness checks reload current Linear truth, while snapshot-based downstream IDs keep unchanged work idempotent.

The current composition still enables triage only, so deployed behavior remains Backlog-only until a Spec or Implement consumer lands.

Validation

• make check
• make smoke-linear-automation
• focused Linear automation tests: 37 passed
• change review: implementation and code quality passed with no findings

[ferueda]

Docs drift review: no concrete drift found. The PR updates the closest architecture and Linear automation docs for route-derived observed states, per-cycle Open readiness checks, snapshot-based work identity, and the current Backlog-only composition.

Verification reviewed: make check and make smoke-linear-automation.

[ferueda]

Live FER-270 smoke: PASS

• Used a disposable Linear project with one Open + Spec target and one unresolved blocker.
• Ran the production poller and readiness router through a fresh local self-hosted Inngest instance with a capture Spec consumer.
• While blocked: no work request.
• Moved only the blocker to Done: the target issue revision remained unchanged and one Spec request was dispatched after the router's confirmation read.
• Polled the unchanged ready snapshot again: no duplicate Spec consumer run.
• Deleted both throwaway issues, the disposable project, local SQLite state, and the temporary smoke driver.

No model was invoked and no existing Harness issue was touched.