chore(meshai): v0.5.5 -- cleanup bundle (gitignore env anchor, ducting health event_count, mesh_sources secret stripping, delete unused SeverityRouter)

Four independent low-risk fixes from the deferred list. Bundled in a single
commit because none are large enough to warrant their own tag and none
touch the safe-mode-sensitive paths (dispatcher / consumer / toggle config).

1) .gitignore: change bare `env/` to `/env/` so the rule anchors at the
   repo root only. The unanchored form was matching `meshai/env/` (the
   adapter package directory) and forced `git add -f` workarounds during
   2.14 / 2.16.1. Verified post-fix: `git check-ignore -vn meshai/env/test.py`
   reports no pattern match; `git check-ignore -v env/foo` still matches
   the new `/env/` rule.

2) meshai/env/ducting.py: health_status.event_count was hardcoded `0`
   from before Phase 2.13 added real event emission. Replaced with
   `len(self._events)`, which is the pattern every other env adapter
   already uses (fires/firms/nws/swpc/traffic/roads511/usgs/usgs_quake/
   avalanche). Flows through env.store.health_status → /api/env/status
   so the dashboard counter starts reflecting reality.

3) meshai/config_loader.py save_section: list-section secret stripping.
   The path landed in C.2.1 fed list items into check_secrets() with
   path="" or with `<field>[<i>]` syntax, neither of which matched the
   `mesh_sources.*.api_token` / `notifications.rules.*.smtp_password`
   regexes in SECRET_FIELDS (where `*` matches a single dotted token).
   Result: a raw secret submitted on a list-section save could slip
   through to the YAML file. Fix uses dotted-index form `<field>.<i>.<key>`
   for both nested-list (notifications.rules) and top-level-list
   (mesh_sources) paths. Also extended _raw_section construction +
   _ondisk_ref to walk list-shaped on-disk YAML by integer index so
   the C.3.1 ${VAR}-placeholder preservation now works for list sections
   too. Three new tests round-trip the mesh_sources placeholder case,
   the mesh_sources raw-secret rejection, and the nested-list
   notifications.rules placeholder case.

4) meshai/notifications/pipeline/severity_router.py: deleted.
   The fork-by-severity routing it implemented was never wired in
   production -- _tee in build_pipeline does the dispatcher+digest
   fanout directly. The class had two test references in
   tests/test_pipeline_skeleton.py that exercised "no matching rule"
   and "unknown severity" paths; those guarantees are now covered by
   tests/test_v052_dispatcher.py (stats counters) and the existing
   Dispatcher-class tests. Removed the file, the __init__.py imports
   and __all__ entries (SeverityRouter + StubDigestQueue both), the
   two test methods, and the docstring mention.

Verification:
- py_compile clean on all four touched modules.
- `grep -rn SeverityRouter meshai/ tests/` returns zero.
- pytest 328 passed (was 327 at v0.5.4; net: -2 SeverityRouter tests,
  +3 secret-preservation tests = +1).
- .gitignore anchor diagnosed via `git check-ignore -vn`.

Safe-mode preserved -- no toggle enabled, no master enabled, no central
enabled, no adapter feed_source flipped.

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

.gitignore

@@ -33,7 +33,9 @@ wheels/
 # Virtual environments
 venv/
 ENV/
-env/
+# v0.5.5: anchor to repo root only -- bare `env/` matched meshai/env/ (the
+# adapter package directory) and forced `git add -f` workarounds in 2.14/2.16.1.
+/env/
 .venv/
 
 # IDE