enh(#2904): add a reviewer entry type so third-party reviewer lanes are discoverable (#2912)

* feat(#2904): add a `reviewer` entry type so third-party reviewer lanes are discoverable

ADR-2782 made a reviewer lane installable by a third party, but neither
discoverability catalog could hold one. The Community Capability Registry
requires a non-empty `loopExtensionPoints` and forbids a lane from declaring
any hook kind, so a `role: "reviewer"` entry is unsatisfiable by construction;
the EoS Registry is for ADR-1239 host integrations, which a lane is not.

Adds a third catalog — `docs/registries/reviewers.json` →
`docs/registries/reviewer-registry.md` — whose `interactions` describes the
lane: slug, flags, transport, evidenceClass, reviewsSection, requiresBinaries,
configKeys, runtimeCompat.

The lane vocabulary is a hand-written mirror of `capability-validator.cjs`
(the same pattern as `AXES` mirroring `HOST_INTEGRATION_AXES`), with parity
enforced by tests/registry-reviewer-parity.test.cjs. `slug` deliberately uses
the runtime `LANE_SLUG_RE` grammar rather than the registry's kebab-only `id`
rule, so real lanes (`lm_studio`, `4o-mini`) are not rejected.

Two binary type branches became three-way Map dispatch. Both now fail loudly
on an unrecognized type instead of silently treating it as a capability —
`renderMarkdown` in particular writes a committed catalog file, so a silent
wrong-title render was the worst failure mode available.

Also fixed while here: `gen-registry.cjs` parsed source JSON with no error
handling, so a malformed or non-array `capabilities.json` surfaced as a raw
SyntaxError/TypeError instead of an actionable CLI error.

Closes #2904

* fix(#2904): bound and sanitize untrusted registry `interactions` strings

Review findings from the pre-PR passes.

Security (isolated pass): `interactions` string fields reached the generated,
committed Markdown catalog with no control-character check and no length
bound. A `reviewsSection` carrying ESC and a `requiresBinaries` element
carrying NUL plus 5000 characters validated clean and landed verbatim in the
rendered page — `mdInline` escapes Markdown metacharacters and collapses CRLF,
but nothing else. The identical gap already existed on the capability type's
`configKeys`/`requires`/`runtimeCompat`/`produces`/`consumes`, so it is fixed
there too rather than inherited into a third type.

`hasDisallowedControlChar` is lifted to module scope so exactly one
implementation exists, and a shared `validateStringArrayField` enforces
control-character rejection, a 200-character element cap and a 50-element
array cap for both types.

Correctness (standards pass): `renderMarkdown`'s per-entry summary builder was
still an if/else-if chain whose final `else` was the capability branch — the
one per-type dispatch point this change had not converted, and the same silent
fallthrough it removes elsewhere. It now lives in `RENDER_META` alongside the
title, so a fourth type cannot silently inherit capability's rendering. All
three types' rendered output is byte-identical to before the refactor.

Also corrects a test comment that still claimed the reviewer suites were
failing-first against an unmodified module.

* chore(#2904): backfill changeset PR number (#2912)

.changeset/merry-seals-climb.md

@@ -0,0 +1,5 @@
+---
+type: Added
+pr: 2912
+---
+**Third-party reviewer lanes can now be listed in a discoverability catalog.** ADR-2782 made a reviewer lane installable by a third party, but the two existing registries could not hold one — the Community Capability Registry requires a non-empty `loopExtensionPoints`, which a lane registers on none of, and the EoS Registry is for host integrations. A new Reviewer Lane Registry (`docs/registries/reviewers.json` → `docs/registries/reviewer-registry.md`) gives lanes a home, with an entry schema describing the lane itself: slug, flags, transport, evidence class, and REVIEWS.md section. (#2904)