feat: add entire stop command

[peyton-alt]

Adds entire stop to manually mark one or more active sessions as ended,
preventing future checkpoint leakage without waiting for the agent to stop
naturally.

Pure state mutation — no writes to the checkpoints branch, no condensation.
The existing PostCommit hook handles any remaining condensation on the next
commit.

Flags: --session , --all (worktree-scoped), -f/--force

Output

Session with a committed checkpoint:
✓ Session 2026-03-19-demo-checkpoint stopped.
Checkpoint: a3b2c4d5e6f7

Session with uncommitted work:
✓ Session 2026-03-19-demo-work stopped.
Work will be captured in your next checkpoint.

Session with no work:
✓ Session 46f9e86a-02c2-46c8-a733-e4323cb5c2cb stopped.
No work recorded.

Already stopped:
Session 2026-03-19-demo-work is already stopped.

Not found:
Session not found.

No active sessions:
No active sessions.

---

Note

Medium Risk
Adds a new CLI command that mutates on-disk session state (marking sessions as ENDED) and includes interactive selection/confirmation paths, so mistakes could prematurely end sessions or affect worktree scoping.

Overview
Introduces a new entire stop command that manually marks session state as ENDED (without writing checkpoints), supporting --session <id>, worktree-scoped --all, and -f/--force to skip confirmation.

Implements interactive selection/confirmation flows (single-session prompt or multi-select when multiple sessions are active), and adds unit tests covering main stop paths and error cases. Documentation is updated in README.md and the changelog to include the new command.

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[Copilot]

Pull request overview

Adds a new entire stop command to manually mark one or more sessions as ended (state-only mutation) to prevent further checkpoint leakage without waiting for the agent lifecycle to naturally close the session.

Changes:

• Introduces entire stop with --session, --all (worktree-scoped), and --force flows, including interactive selection when multiple sessions exist.
• Adds unit tests covering the non-interactive stop flows and common error cases.
• Documents the new command in README.md and CHANGELOG.md, and wires it into the root command.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
README.md	Adds entire stop to the command list.
cmd/entire/cli/stop.go	Implements the stop command logic (single/all/multi-select) and output messaging.
cmd/entire/cli/stop_test.go	Adds unit tests for stop behavior (force, flags, not-found, non-git repo, etc.).
cmd/entire/cli/root.go	Registers the new stop subcommand.
CHANGELOG.md	Notes the addition of entire stop.