Resume the most recent session for a repo, branch, folder, or current dir

[jongio]

Problem

dispatch open --last resumes the single most recently active session across everything, and dispatch open <id> resumes an exact session. Neither helps the common case: "resume the last session I had in this repo" or "in this branch". Today you have to run dispatch search --repo owner/repo --ids, read the first line, and pass it back to dispatch open. That is two commands and a copy/paste for something people do many times a day, and it is awkward to bind to a shell alias.

flowchart TD
    A[dispatch open --current] --> B[Detect git repo and branch from cwd]
    A2[dispatch open --repo/--branch/--folder] --> C[Build FilterOptions]
    B --> C
    C --> D[List sessions, updated desc, limit 1]
    D --> E{Match found?}
    E -->|yes| F[Resume with existing launch path]
    E -->|no| G[Clear error, exit non-zero]

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Proposed solution

Teach dispatch open to resume the most recently active session that matches a filter, reusing the same filter fields as dispatch search and the same git detection as the --current startup filter:

• dispatch open --current resumes the newest session for the current git repo and branch (read from the working dir, or --cwd <path>).
• dispatch open --repo owner/repo, --branch <name>, and --folder <path> resume the newest session matching those filters, and can be combined.
• All of these keep working with --mode and --print, so dispatch open --current --print writes the resume command without launching.
• When no session matches, print a clear message and exit non-zero (do not fall through to launching something unrelated).

This complements --last rather than replacing it. --last stays the "newest anywhere" shortcut; the new filters scope it.

Acceptance criteria

• dispatch open --repo owner/repo resumes the most recently active session for that repo.
• dispatch open --branch <name> and --folder <path> scope the same way and can be combined with --repo.
• dispatch open --current resumes the newest session for the git repo and branch detected from the working dir, honoring --cwd.
• --mode and --print work with the scoped forms.
• A non-git dir with --current, or no matching session, prints a helpful error and exits non-zero.
• --last still behaves as before and cannot be combined with the scoped filters (clear error if it is).
• Help text and completion list the new flags; unit tests cover match, no-match, and flag-conflict cases.

Notes

The git repo/branch detection already exists for the --current startup filter in startup.go. Session selection can reuse the ListSessions path with SortByUpdated/Descending and limit 1, the same ordering defaultOpenGetLastSession uses.