fix(git-poll): stop the PR/CI poller burning the GitHub API budget

Measured on a 94-project workspace with 11 projects visible: the daemon
was spawning 108-300 `gh` invocations per minute across 34 repositories,
or roughly 4,600-9,300 REST requests per hour against a 5,000/hour limit
(plus 2,000-3,000 GraphQL). The limit was exhausted and every PR/CI badge
was quietly getting 403s.

Five compounding causes, all fixed here:

1. Scope. `gh` followed the *streaming* set — visible projects plus any
   owning a terminal a remote client subscribed to. Clients subscribe to
   every terminal in the daemon's state, not just what they render, so a
   project merely owning a terminal landed on the responsive GitHub
   cadence: 34 repos polled to render 11 badges. `gh` now follows
   visibility only; local git status keeps the wider set.

2. Cadence. One repo with running CI flipped a global flag that put all
   34 onto the 15s cadence. Cadence is now per project, via
   `GithubPollSchedule`.

3. Redundancy. Checks on a commit only move while something is running,
   but every poll re-fetched them regardless. `fetch_ci_checks` now takes
   the commit a settled result describes and skips the request while the
   branch still points at it — a repo parked on `main` costs one local
   ref read instead of two REST calls. Settled results are revalidated
   every ~10min so a re-run is still noticed.

4. Overlap. A pass ran projects strictly sequentially (~250ms each, so
   ~17s for 34 repos) while being re-spawned unconditionally every
   cadence tick — passes stacked, and the same repo was observed fetched
   twice in the same second. Passes now run 4-wide (matching the process
   bus's Poll lane) and never start while one is in flight.

5. Refusals. Nothing recognised a rate-limit response, so an exhausted
   budget changed nothing about the request rate. A refusal now parks all
   `gh` traffic with a doubling 60s-30min backoff, and is no longer
   cached as "this project has no PR".

Also drops the CI cache on every local commit: checks belong to the last
*pushed* commit, which a local commit doesn't move, so that eviction only
blanked the badge and forced a refetch.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SfU7oDTtKEaQ9vG7Y6fquX

Author: matej21