feat: add multi-repository review picker

[ileadall42]

Problem

A Herdr workspace can contain agent panes from several Git repositories, especially when a top-level project is composed of multiple checkouts. The existing review actions intentionally follow the focused pane, but that makes it easy to review the wrong repository/base or send comments to the wrong agent when several repositories are active.

Changes

• add review:pick for explicit repository, base branch, and feedback-agent selection
• add send-review:pick so an open review can be sent to any live agent selected at delivery time
• discover Git worktrees and agents from every pane in the current Herdr workspace, preferring each pane's foreground cwd
• open Hunk with an explicit <base>...HEAD range and persist the chosen agent association for later send-review
• use dependency-free numbered overlay pickers, including POSIX and Windows pane entries
• preserve the existing focus-based review actions unchanged
• document the workflows and cover discovery, selection, branch refs, pane reuse, delivery, association, manifest, and keys with tests

The new send-time picker has no plugin-wide default key because likely prefix+alt+... candidates can collide with Herdr defaults. Users with a freed key can bind jhochenbaum.hunkdiff.send-review:pick explicitly.

Validation

• npm run check
• npm run build
• npm test (28 files, 686 tests)
• npm audit --audit-level=high --registry=https://registry.npmjs.org/ (0 vulnerabilities)
• installed the branch as a real Herdr plugin and invoked review:pick successfully

Related to #15: the picker makes the comparison base explicit instead of assuming a single repository/base for the whole workspace.