chore: remove vendored session-sentinel/ (unique work salvaged upstream) (#107)

## Summary
Removes the embedded `session-sentinel/` directory as part of the
2026-05-26 estate duplicate-reconciliation pass.

The only content unique to the vendored copy was:
- `.eph` files in `src/core/` (WIP Ephapax port)
- an `assail:` Justfile recipe

Both have been salvaged into the canonical
`hyperpolymath/session-sentinel` repo on branch
`wip/ephapax-port-from-ambientops` (companion PR:
hyperpolymath/session-sentinel#17).

## Rationale
The vendored copy was diverging from canonical (different SHAs for
equivalent commits, plus stale `.as.old` backup files), so removing it
eliminates a future drift hazard. Ambientops continues to consume
session-sentinel as an external dependency; nothing in ambientops
currently imports from the vendored tree.

## Test plan
- [x] No imports of `session-sentinel/...` in ambientops
Rust/AffineScript code (verified via grep before commit)
- [ ] CI green

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath