extracts the deepen-retry helper from `GitFetchTool` into shared
`$gitFetchWithDeepen` and applies it to every fetch in `checkoutPrBranch`
(baseRef, pull/N/head, before_sha temp branch). on shallow clones with
deep PR ancestry — the failure mode behind ~10 of 51 `heuristic:very-slow`
runs in 24h on `remotion-dev/remotion` — the baseRef fetch was throwing
`Could not read <sha>` to the agent before the compare-api deepen block
could run. agents then burned 10+ minutes retrying `checkout_pr` and
falling back to ad-hoc shell `git fetch --deepen` workarounds.
also splits the analyzer's `heuristic:git-error-recovered` into
`heuristic:git-shallow-unreachable` and `heuristic:git-shallow-lock`
buckets so future audits surface this without manual log-grep.
closes#656.
Colin McDonnell
4ad649ebb9