Session bootstrap: present resume/new/review choice instead of auto-continuing

## Problem

Reopening loom in a directory that has a prior session silently auto-continues the last task ("nice to see you back, let's continue") instead of giving the user a choice. User feedback:

> if i close and reopen it didnt just start the llm going on the last task again. i closed, changed model, opened again and it just went 'nice to see you back, lets continue' or some similar thing and kept right on. id like to be able to actively manage a session/ task.

This is especially painful when the user changed model between sessions -- that's exactly the moment you want a decision point, not a silent continuation.

## Expected behavior

When loom opens into a directory with prior session state, `extensions/loom/session-bootstrap.ts` should present:

1. **Resume** the previous task where it left off
2. **Start new** session in this directory (archive or set aside the previous)
3. **Review** what's queued / was in progress before deciding

This is the intent of session-bootstrap per CLAUDE.md; current behavior doesn't match.

## Notes

- "Review" could just be a summary of the last N messages + any pending tool calls
- Model change between sessions is a strong signal the user wants a checkpoint -- could bias toward prompting harder in that case

Read CLAUDE.md and extensions/loom/session-bootstrap.ts first to compare the intended bootstrap flow with the current automatic continuation. Trace how prior session state is loaded and how the opening prompt is presented. Done means reopening a directory offers Resume, Start new, and Review choices, including a useful summary for Review.

Written by the indexing model from the issue text.