refactor(#1756): derive getDirName from a documented runtime.localConfigDir descriptor axis (#1757)

ADR-1239 Phase B (parent #1679). getDirName was a hand-maintained 15-branch
if-chain mapping each runtime to its local content-rewrite dot-dir. Relocate
those values into a documented runtime.localConfigDir descriptor field; derive
getDirName from registry.runtimes[id].runtime.localConfigDir (fallback .claude).

- 16 capability.json gain runtime.localConfigDir (byte-identical values)
- capability-validator.cjs requires it (non-empty dot-dir); registry regenerated
- docs/reference/capability-manifest.md documents the field + the three
  divergent values (copilot=.github, antigravity=.agents, kimi=.kimi-code)
- drift-guard test: golden value map + key-set equality both ways

Byte-identical install output for all 16 runtimes (golden-parity harness #1730).

Closes #1756

Co-authored-by: review-bot <review-bot@gsd>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

docs/reference/capability-manifest.md

@@ -143,6 +143,7 @@ Runtime capabilities describe how GSD projects its artefacts onto one host CLI.
 | Axis | Field | Type summary |
 |---|---|---|
 | Config home | `runtime.configHome` | Structured object with `kind` (`dot-home` \| `dot-home-nested` \| `xdg` \| `generic-agents-root`), `name`, optional `parent`, `env[]`, `probe[]`, `probeExists`, `skillsHome`. `probeExists` is an optional sub-path applied to probe candidates: for `generic-agents-root` it is a hard filter (a candidate qualifies only if `<candidate>/<probeExists>` exists); for `dot-home-nested` it is a preference that makes probing pick the candidate GSD owns (e.g. `gsd-core/VERSION`) over a bare-existing sibling before falling back — see ADR-1016 and #213/#217. |
+| Local config dir | `runtime.localConfigDir` | Required dot-prefixed string. The runtime's **local** content-rewrite directory — the `./` target GSD stamps into rewritten artefact bodies (e.g. `./.claude/` → `./<localConfigDir>/`) and the local install dir basename. Backs `getDirName()` (registry-derived, #1679). Usually `.<runtime>` (the runtime's home dot-dir), but **three runtimes diverge** because they read GSD's content from a non-home directory: `copilot` → `.github` (GitHub Copilot reads custom instructions from `.github/copilot-instructions.md` / `.github/instructions/`; see `convertClaudeToCopilotContent` rewrites in `src/runtime-artifact-conversion.cts`), `antigravity` → `.agents` (local agent/workflow dir; see the antigravity rewrites in `src/runtime-artifact-conversion.cts`), `kimi` → `.kimi-code`. Distinct from `configHome.name` (the **global** install home, which for these three is `.copilot` / `antigravity` / `agents`). Byte-parity-proven against the prior hand-maintained mapping by the golden-install-parity harness. |
 | Config format | `runtime.configFormat` | Closed enum: `settings-json` \| `toml` \| `markdown` \| `markdown-dir` \| `none`. |
 | Artefact layout | `runtime.artifactLayout` | Object with `global` and `local` arrays of `ArtifactKind` (`kind`, `destSubpath`, `prefix`, `nesting`, `recursive`, `stage`). |
 | Command style | `runtime.commandStyle` | Closed enum: `slash-hyphen` \| `shell-var`. |