Replace native-suite-count file with dynamic test discovery (#11413)

* Derive the native suite count on the fly instead of registering it in a file

test/native-suite-count was a manually-maintained register of the test_*
directory count, reconciled against the actual directories by
bin/run-tests.sh (as an AMBER verdict) and by a dedicated suite-count-check
CI job. The reconciliation only ever guarded the file itself: the check
that matters - suites that actually ran vs. the test_* directories on
disk - already derives its expected count from a directory walk, so the
file added a bookkeeping step to every suite addition/removal without
adding signal.

Remove the file and everything that existed to keep it honest:

- bin/run-tests.sh: drop the canonical-count file read, the count-mismatch
  AMBER verdict, and the [canonical: x/y] suffix; the verdict lines already
  carry ran/expected from the directory walk. The shuffle seed suffix stays.
- test_native.yml: delete the suite-count-check job and its needs: edges.
- Docs (copilot-instructions.md, AGENTS.md, test/README.md) and the
  test-script comments now describe the count as derived from test/test_*
  at run time.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01APCEfNjd1X7ErDHEzT6Dqd

* Add suite-shrinkage-check: fail a PR that silently loses a test_* suite

With test/native-suite-count gone, nothing in CI noticed the suite set
shrinking: platformio test discovers and runs whatever test_* directories
exist, and bin/run-tests.sh derives its expected count from the same walk,
so a suite directory lost in a bad rebase or an overzealous cleanup just
means fewer suites run - every remaining check stays green.

Restore that tripwire git-aware instead of file-based: on pull_request
runs, compare the test_* directory list at the PR's merge base against the
PR result. A vanished suite fails the job unless its name appears in the
PR title, PR body, or a commit message in the PR's range - a deliberate
removal satisfies that by stating what it removes; an accidental loss
cannot. Other events skip: they have no natural base, and PRs are where
accidents arrive. No job depends on this one (a skipped job would skip
its dependents).

Incidentally: test/ currently holds 47 test_* directories while the
deleted count file said 46 - the manual register had already drifted,
which is exactly the bookkeeping failure mode this replaces.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01APCEfNjd1X7ErDHEzT6Dqd

* Re-pad the verdict table after shortening the AMBER row

Shrinking the AMBER cell left the table's column padding inconsistent,
which trunk (prettier + markdownlint MD060) rejects.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01APCEfNjd1X7ErDHEzT6Dqd

---------

Co-authored-by: Claude <noreply@anthropic.com>

test/README.md

@@ -467,8 +467,8 @@ Unity suite, because what it asserts - the exit status and printed report of
 `meshtasticd --check`, and the fact that a normal run still refuses a bad config - are
 properties of the process, not of a linkable function. Fixtures live in
 `test/fixtures/portduino-config/` (see the README there); CI runs it in
-`test_native.yml`. It is not counted in `native-suite-count`, which only tracks `test_*`
-directories.
+`test_native.yml`. It is not a `test_*` directory, so it sits outside the suite count the
+harness derives from `test/`.
 
 ```bash
 pio run -e native && ./bin/test-config-check.sh
@@ -476,10 +476,11 @@ pio run -e native && ./bin/test-config-check.sh
 
 ## Existing Test Suites
 
-**This table is a description, not an inventory.** The canonical suite total lives in
-`test/native-suite-count`, is machine-checked against `test/test_*` on every full run and in CI
-(`test_native.yml`), and is the only number that should be trusted or quoted. Entries below carry
-per-suite descriptions the count cannot; do not infer completeness from the row count.
+**This table is a description, not an inventory.** The canonical suite total is the number of
+`test_*` directories under `test/`, detected on the fly by `bin/run-tests.sh` on every full run
+and cross-checked against the suites that actually ran. That derived count is the only number
+that should be trusted or quoted. Entries below carry per-suite descriptions the count cannot;
+do not infer completeness from the row count.
 
 | Suite                        | Module Under Test             |
 | ---------------------------- | ----------------------------- |

test/native-suite-count

@@ -1 +0,0 @@
-47