fix(kyberforge): scope corroboration to a real sentence boundary

A prose-form routing target blocks a commit only when its own sentence names
another target that resolves. That makes the sentence splitter part of the
ADR-0020 contract rather than an implementation detail, and the naive
"period, space, capital" rule got it wrong in both directions:

- OVER-SPLIT: `e.g. "..."` is not a sentence end, but the quote looks like a
  start. The clause was cut in half and the corroborator stranded on the far
  side, so a genuinely dangling target silently demoted to SUGGESTION — a
  measurement taken and then discarded, the vacuous-green shape this gate
  exists to prevent. Seven such splits are live in the current corpus.
- UNDER-SPLIT: a sentence opening with a code span or a lowercase skill name
  was not seen as a start, so two sentences merged and a resolving target
  vouched for an unresolvable one it never stood beside — a hard FAIL with no
  escape hatch, which is the exact failure corroboration was added to prevent.

The splitter now excludes the five abbreviations that occur in routing prose
and admits a backtick or lowercase letter as a sentence opener. Applied
byte-identically to all three copies of the shared resolver.

Verified zero-delta against the corpus: 37 ERROR / 58 SUGGESTION / 2 dangling
before and after, findings byte-identical. The exposure this closes is to the
descriptions #99 is about to rewrite, not to the ones already measured — which
is why the deferral reason recorded on PR #103 ("can move the documented corpus
counts") does not hold and the fix lands here rather than after the retrofit.

Three regression tests, one per direction plus the backtick opener, each proven
non-vacuous by reverting the splitter alone and watching it go red.

Refs: #99
ADR: 0020

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/adr/0020-skill-description-and-body-context-contract.md

@@ -129,6 +129,14 @@ clause**, and a **boundary clause**. Capability enumeration, output-format detai
   dependency, and none needs to. Verified: a tree holding only `plugins/` and the root `apm.yml`,
   with no `.claude/` or `.agents/` anywhere, now produces findings identical to the working tree —
   26 description FAILs, 9 body FAILs, 2 dangling targets, 0 missing references, 58 SUGGESTIONs.
+- **Blocking is scoped to a sentence, which makes sentence boundaries load-bearing.** A prose-form
+  target earns a hard error only when its own sentence names another target that *resolves*; route
+  notation (`/name`, `→ name`) is exempt and always blocks. So the splitter is part of the contract,
+  not a detail of it. `e.g. "…"` is not a sentence end, and a sentence opening with a code span or a
+  lowercase skill name is a start; getting either wrong moves targets between the two tiers in
+  opposite directions — a stranded corroborator silently demotes a real finding to SUGGESTION, and a
+  missed boundary lets one sentence vouch for a target it never stood beside, producing a hard FAIL
+  with no escape hatch.
 - **The blanket pushiness rules are deleted.** `skill-author/SKILL.md:104` and
   `description-quality.md:21` are replaced by a conditional: add an indirect trigger only where the
   user's natural phrasing genuinely omits the domain word — true for the `gitea-*` family, false for