Add dispatch path to print a session working directory for cd

[jongio]

Problem

Jumping into a session's working directory from the shell is clumsy. dispatch info --json includes the directory, but you have to pipe it through jq to get a bare path, and dispatch open --print prints the resume command, not the folder. There is no single command that resolves a session to just its directory.

Solution

Add dispatch path that prints one session's working directory and nothing else, so it drops straight into a subshell:

cd "$(dispatch path my-alias)"

It resolves the session the same way dispatch open does: by full ID, alias, short ID prefix, --last (most recent), or --current (auto-detect from the working directory). It reuses the existing resolver so the matching rules stay identical.

Output is the absolute path on stdout with a trailing newline. If the session has no recorded directory, or the directory no longer exists on disk, it prints a clear message to stderr and exits non-zero so cd "$(...)" fails loudly instead of landing in the wrong place.

Acceptance criteria

• dispatch path <id> prints the session's absolute working directory and exits 0
• Resolution supports alias, short ID prefix, --last, and --current, matching dispatch open
• A session with an empty directory exits non-zero with a message on stderr
• A directory that no longer exists exits non-zero with a distinct message on stderr
• Ambiguous or unknown references produce the same errors dispatch open gives
• Unit tests cover each resolution path and both error cases
• README and dispatch man document the command