feat(fleet): make dashboard key hints respect node state

[outofcoffee]

The dashboard's key help now shows start and stop only for the node where they would drive something, the way the keep and abandon entries already do.

Summary

• Grid and detail footers build their entries from per-key offers (startOffered, stopOffered) that read the board's current read of the node under the cursor, instead of a static list with only keep and abandon gated
• Start is offered where the read does not report the node running, stop where it does, and neither while an action is in flight; a failed or absent read offers start but not stop
• Drop the old dashFooterHints path; both screens pass their built line to footerLine
• The keys themselves stay ungated: pressing one the hint hides still makes the call, and the node's own refusal lands on the status line
• Key table in docs/commands/fleet.md carries the same shown-only-where-they-drive-something wording as the keep row

Implementation details

The offer predicates live beside keepOffered/canAbort on the model and share a small nodeRunning check (read OK and state running). Gating is display-only and deliberately does not touch the key handlers, so a start pressed on a running node is still refused by the engine and reported, matching the one-shot command's behaviour. OpenSpec change state-aware-dashboard-keys records the spec delta and design.