obs: scheduled-lane health, freshness telemetry, and standard artifact envelope

[thymikee]

Part of #1412 (Track E — gate integrity). From external review: nightly/weekly lanes (replays, perf, conformance differential, and the upcoming fuzz/mutation/torture/model-bench lanes) can fail or stop running for weeks while PR CI stays green. The observatory must watch the watchers.

Deliverables

• Standard artifact envelope for every scheduled lane: schemaVersion, commit SHA, tool/config versions (or content hashes), seed where applicable, duration, result. Existing nightlies adopt it as they are touched; new lanes (test: nightly fuzz lane — parser inputs must fail as typed AppErrors, never hang #1414, test: weekly mutation-score ratchet over decision kernels (Stryker, non-gating first) #1415, test: seeded concurrency torture lane for session/lease/lock invariants #1416, ci: weekly scheduled model-backed help-bench run with pass-rate deltas #1427) MUST ship with it.
• A scheduled health job: reads last-run/last-success for every schedule:-triggered workflow via the GitHub API (the workflow list is derived from .github/workflows/, not hand-maintained) and posts/pings a tracking issue when any lane misses or fails two consecutive cadences.
• Retry telemetry feeding the repo-health snapshot (obs: repo-health snapshot command aggregating existing analyzers into one JSON #1423) and delta comment (obs: evolve the size-report PR comment into a quiet quality-delta comment + metric history on main #1424): retry count per run and retry-eligible-file count (ci: single-retry policy for enumerated contention-flaky files (timeouts only) #1419) as ratcheted metrics — the flake list shrinking is the goal, and growth must be visible.
• Timeout classification from structured Vitest reporter metadata, never error-text sniffing (the classifier lessons: categories from recorded fields, not parsed names).
• Branch-protection required-contexts audit lives here if it needs elevated API scope (see the gate-manifest sibling issue).

Acceptance

• Simulate a lane going dark (disable a test schedule twice): the health job opens/pings the alert issue.
• Every scheduled artifact produced after this lands carries the envelope; obs: repo-health snapshot command aggregating existing analyzers into one JSON #1423's snapshot exposes lane freshness fields.