feat: list and cd with fuzzy matching #3

Goal

Every managed repository can be found and jumped to by a partial name.

Context

• Trigger: third milestone, consumes the Repo value and the resolver from the identity issue.
• Affected: internal/scan, internal/shell, cmd/gitcd.

A linked worktree's .git is a file, not a directory. scan treats a .git file as a
repository boundary and stops descending, which covers linked worktrees and .claude/worktrees/
in one rule rather than two special cases.

The picker is a numbered list on stderr reading from stdin. An fzf integration is deliberately
deferred until the built in one has been used for a while.

Acceptance criteria

• internal/scan walks root into Repo values and stops descending at a repository
  boundary, with a test covering a linked worktree.
• gitcd list prints every repository under root.
• gitcd list --json prints the same as structured output.
• gitcd list --worktrees includes linked worktrees, which are hidden by default.
• gitcd cd <query> resolves a unique partial match to a path on stdout.
• An ambiguous query with a terminal on stdin shows a numbered picker on stderr.
• An ambiguous query without a terminal exits non zero and lists the candidates on stderr.
• A query matching nothing exits non zero with a message naming the query.
• Shell completion for cd in zsh and fish, shipped through shell-init.
• Tests cover matching, ambiguity and the non terminal path.
• task check passes.

Out of scope

• fzf integration.
• doctor, adopt, status, sync.
• Indexing repositories lying outside root.