[TASK] git.worktrees: FastWalker(.git) discovery

[kirich1409]

Context

Walk projectRoots via FastWalker(markers: [.directory(".git")]). Emit one candidate per repo.

Acceptance criteria

• All git repos inside roots discovered.
• .git not descended.

Files to touch (estimate)

best-effort estimate; agent refines during implementation

Implementation notes

n/a

Dependencies

• Blocked by [TASK] FastWalkerImpl via getattrlistbulk (P1) #58
• Blocked by [TASK] Exclusion split (basename + pathPrefix with canonicalize) #60

Safety checklist

See .github/ISSUE_TEMPLATE/task.yml — tick applicable invariants in the issue if any destructive path is touched.

Definition of Done

• Acceptance criteria satisfied and verifiable
• Tests added or updated
• No build / lint / test regressions
• Self-review against research-report invariants
• Code review approved