refactor: one walk over root, not two #15

Goal

One walk over root, used by every command that needs the repositories under it.

Context

• Trigger: feat: list and cd with fuzzy matching #3 and feat: status, sync and open across repositories #5 were implemented in parallel and both needed the same walk. feat: status, sync and open across repositories #5 was told to
  build the minimum it needed rather than block on feat: list and cd with fuzzy matching #3, so the duplication is deliberate and was
  recorded in AGENTS.md and in both pull request bodies. It was never meant to survive.
• Affected: cmd/gitcd/discover.go, already on main through feat: status, sync and open across repositories #14, and internal/scan, which
  arrives with feat: list and cd with fuzzy matching #3.

discoverRepos returns a path list. scan.Walk returns Result values carrying the richer
profile.Repo, and since #3's rework it also asks git for each repository's own linked
worktrees, which discoverRepos does not. So the two already disagree: path, open, status
and sync cannot see a worktree that list and cd can.

Acceptance criteria

• internal/scan is the only walk over root in the repository.
• cmd/gitcd/discover.go is gone, and path, open, status and sync use
  internal/scan.
• A linked worktree nested at <repo>/.claude/worktrees/<branch> is reachable from every
  command that takes a repository query, not only from list and cd.
• Whether each command includes linked worktrees by default is stated once and tested, rather
  than decided separately per command as it is today.
• A nested independent repository is still never reported, with the existing test kept.
• AGENTS.md no longer carries the reconciliation note.
• task check passes.

Out of scope

• Changing the boundary rule itself, or how profile.Inspect resolves a repository.
• Any new command.