Widen agent step-cap margin to 10 min (0.2.8): cold setup can't cancel the job

[dpup]

Problem

The Run-agent step cap is derived as job − AGENT_STEP_FALLBACK_MARGIN, so an over-running agent fails its step (job stays un-cancelled, the !cancelled() salvage still commits/pushes/opens the PR) instead of tripping the job cap (#12545). But that one margin has to clear two things the step cap doesn't:

1. the Setup agent toolchain step that runs before the agent — a cold-cache toolchain install is ~7 min; and
2. the commit/push/open-PR salvage after it.

At a 5-min margin, a cold setup (>5 min) means the agent step can never reach its own cap before the job cap fires → the job is cancelled mid-agent and the salvage is skipped. Observed downstream as #12611 (sentry_sweeper cancelled at 1/3 on a cold run). Separately, the tight derived step cap cut off prose-heavy scheduled lanes before they could self-verify (a retro at job=20 → step=15 timed out and published an unverified note that then thrashed review).

Fix

• AGENT_STEP_FALLBACK_MARGIN 5 → 10 — covers a ~7-min cold setup with ~3 min of salvage headroom, so the agent reliably fails its step rather than the job.

• Raise the three agent-step lane defaults by the same +5 so every derived step cap is unchanged, only the setup headroom grows:

  lane	job default	derived step cap
  producer	30 → 35	25 (unchanged)
  pr-fix	30 → 35	25 (unchanged)
  scheduled-agent	15 → 20	10 (unchanged)

Lanes without an agent step cap (analyst/pr-review, merge-gate, fan_in) are untouched.

Non-regressive: any consumer relying on defaults keeps the exact same step caps and simply gains 10 min of setup headroom. Consumers raise timeout= to give the agent more working time — the margin stays 10. (Downstream, the range harness will bump retro to timeout=30 for a 20-min step, resolving the unverified-retro class.)

Tests / docs

• test/agent-step-timeout.test.ts: updated job caps (step caps unchanged) + a new assertion that job − step === 10 across producer/pr-fix/scheduled-agent.
• Full suite green (432 passing).
• Documents the previously-undocumented timeout= attr and the margin semantics in skill/reference/node-types.md.
• Version bumped to 0.2.8 (adjust if your release flow bumps separately).