fix(hooks): scope stop-drift-guard to the toolkit repo via intrinsic marker

[notque]

Problem

The stop-drift-guard.py Stop-event hook is registered at user scope
(~/.claude/settings.json), so it fires in every repo the user opens. Two of
its three checks (smoke-test-hooks, validate-doc-counts) ran against the
session's own repo, so in unrelated repos they found no toolkit doc-count
claims and emitted false-positive drift — the Toolkit drift guard found drift to review advisory rewake seen in e.g. the Hermez OpenStack repo (? / actual
? placeholders).

Fix

Gate handle_stop on repo identity: run only when cwd/pyproject.toml
declares name = "vexjoy-agent". Non-toolkit repos exit 0 before any check
runs.

The marker is intrinsic and committed, so it travels with any clone regardless
of checkout path, git remote, or machine:

Candidate marker	Clone-portable?
Absolute path	No — breaks on clone elsewhere
Git remote	No — breaks on fork/mirror
Per-machine env var	No — not in the clone
pyproject.toml [project].name	Yes

Python 3.10 has no tomllib; the name is read with a MULTILINE regex line
scan, fail-open on any OSError. The VEXJOY_DRIFT_GUARD_DISABLE kill switch,
async_rewake advisory/never-block behavior, and fail-open contracts are
unchanged. Hook version 1.0.0 -> 1.1.0.

Behavioral proof

cwd	_is_toolkit_repo
vexjoy-agent	True (guard runs)
hermez	False (silent no-op)
/tmp, None, ""	False

Tests

+12 tests (TestIsToolkitRepo, TestToolkitRepoGate including the
false-positive regression test). Target suite: 40 pass. ruff check +
ruff format --check: clean.

Note: 3 unrelated pre-existing failures in test_sync_to_user_claude.py /
test_posttool_bash_injection_scan.py (env-specific /tmp path tests) fail
identically on main and touch neither changed file.