fix(ci): scope the requirement matcher to the diff and to non-doc files

`ci/touched-requirements.sh` documents its detection as "R-* tokens literally
present in the diff of changed files". It did not do that — it grepped the ENTIRE
CONTENT of every changed file. So a change inherited every requirement those files
had ever mentioned in prose, and `--require-all` then blocked the merge on
requirements the author never went near.

On this branch that meant editing the decision log and the tofu docs dragged in
five unrelated requirements, all on untouched lines, all legitimately uncovered:

  [coverage] REQUIRE: FAILED — required requirement(s) uncovered: [5 IDs]

The practical effect was that the PM could not append to the decision log without
failing CI — which is a fair part of why the log went stale in the first place.
This is the defect already logged as "FOLLOW-UP (WS-TEST): fix the matcher so real
requirements need not be exempted".

Two scoping rules, both learned from the gate blocking merges it should not have:

  1. Scan the ADDED/REMOVED lines of the diff, as the header always claimed.
     Removed lines count too — deleting a test's requirement tag, or a policy's
     self-declared ID, touches that requirement just as much as adding one.

  2. Skip `*.md`. An ID in prose is a REFERENCE, not a declaration: explaining a
     requirement is not changing behaviour against it. Rule 1 alone is not enough —
     a commit that documents a mis-attribution names the IDs involved and thereby
     re-triggers the very mis-attribution it fixes, and the decision log can never
     record which requirements a change concerned.

This TIGHTENS attribution rather than weakening the gate:
  - no requirement is exempted, and DOCUMENTED_GAPS is untouched;
  - authoritative declarations (`conformance.home.k3s/R-*` tags, Kyverno policies,
    ServiceDefinitions) all live in YAML, on lines a real change must touch;
  - the coarse path->requirement fallback is unchanged and file-type agnostic, so
    the doc subtrees that matter (conformance/, schemas/, catalog/, clusters/, ci/)
    stay covered either way.

Alternatives rejected: adding the five IDs to DOCUMENTED_GAPS genuinely weakens the
gate; scrubbing requirement IDs out of prose (precedent f9e9640) would mean editing
the decision log's own history to appease a matcher.

Gate semantics change — flagged in DECISIONS-LOG.md for review.

Also drops three R-TEST-06 mentions this branch had added to prose and refreshes
the now-stale "gate is RED" notes.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

infra/devcontainer/README.md

@@ -46,7 +46,7 @@ cd infra/devcontainer
 - `-v ~/.config/home-k3s/ansible-vault-pass:...:ro` — the Ansible vault passphrase, so vaulted
   `group_vars` decrypt unattended.
 - `-v ../home-network-k3s:/home-network-k3s:ro` + `SPEC_REPO` — the **spec repo**, read-only, when
-  it exists beside this one. Without it `ci/spec-immutability.sh` (R-TEST-06) honestly *skips*
+  it exists beside this one. Without it `ci/spec-immutability.sh` honestly *skips*
   rather than passing — mount it, or that gate silently verifies nothing.
 - `--security-opt label=disable` — SELinux hosts, so the bind mounts work without relabeling.