refactor(bin): move non-spec root files into references/ across four skills

grill-with-docs, improve-codebase-architecture, tdd, and triage kept
non-spec markdown files at their skill root, in violation of
skill-audit's file-structure.md rule (only SKILL.md/README.md belong
at the root; everything else lives in scripts/, references/, assets/
or tests/). A root-level file is invisible to the ADR-0020
dangling-reference gate, which only resolves unqualified
`references/...` pointers.

- Moved and renamed to lowercase-kebab-case under references/:
  grill-with-docs (ADR-FORMAT.md, CONTEXT-FORMAT.md),
  improve-codebase-architecture (DEEPENING.md, INTERFACE-DESIGN.md,
  LANGUAGE.md), tdd (five files, casing was already fine), triage
  (AGENT-BRIEF.md, OUT-OF-SCOPE.md).
- Updated every in-skill link to the new references/ paths, including
  link text that still showed the old uppercase filenames.
- Fixed improve-codebase-architecture/SKILL.md's cross-skill citation
  of grill-with-docs's two files to the sanctioned possessive form
  with the references/ segment included.
- Updated all four skills' README.md file tables to match.
- Regenerated the flat content mirror via
  scripts/sync-plugin-content.sh --all.

Fixes #122.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PDj6F7SPXzh3FtPN78dZ88

plugins/bin/skills/improve-codebase-architecture/SKILL.md

@@ -33,7 +33,7 @@ Key principles:
 - **The interface is the test surface.**
 - **One adapter = hypothetical seam. Two adapters = real seam.**
 
-If a term or principle above is ambiguous in the case in front of you, or you need the definitions and the principles the two lists leave out, read `LANGUAGE.md`.
+If a term or principle above is ambiguous in the case in front of you, or you need the definitions and the principles the two lists leave out, read `references/language.md`.
 
 This skill is _informed_ by the project's domain model. The domain language gives names to good seams; ADRs record decisions the skill should not re-litigate.
 
@@ -74,7 +74,7 @@ Once the user picks a candidate, drop into a grilling conversation. Walk the des
 
 Side effects happen inline as decisions crystallize:
 
-- **Naming a deepened module after a concept not in `CONTEXT.md`?** Add the term to `CONTEXT.md` — same discipline as `grill-with-docs`, in the format `grill-with-docs`'s `CONTEXT-FORMAT.md` defines. Create the file lazily if it doesn't exist.
+- **Naming a deepened module after a concept not in `CONTEXT.md`?** Add the term to `CONTEXT.md` — same discipline as `grill-with-docs`, in the format `grill-with-docs`'s `references/context-format.md` defines. Create the file lazily if it doesn't exist.
 - **Sharpening a fuzzy term during the conversation?** Update `CONTEXT.md` right there.
-- **User rejects the candidate with a load-bearing reason?** Offer an ADR, framed as: _"Want me to record this as an ADR so future architecture reviews don't re-suggest it?"_ Only offer when the reason would actually be needed by a future explorer to avoid re-suggesting the same thing — skip ephemeral reasons ("not worth it right now") and self-evident ones. See `grill-with-docs`'s `ADR-FORMAT.md`.
-- **Want to explore alternative interfaces for the deepened module?** Read `INTERFACE-DESIGN.md`.
+- **User rejects the candidate with a load-bearing reason?** Offer an ADR, framed as: _"Want me to record this as an ADR so future architecture reviews don't re-suggest it?"_ Only offer when the reason would actually be needed by a future explorer to avoid re-suggesting the same thing — skip ephemeral reasons ("not worth it right now") and self-evident ones. See `grill-with-docs`'s `references/adr-format.md`.
+- **Want to explore alternative interfaces for the deepened module?** Read `references/interface-design.md`.