refactor(#1679): ADR-1239 Phase B — collapse getConfigDirFromHome chain into getGlobalConfigHomeFragment [AC2 slice 2/6] (#1801)

* refactor(#1679): ADR-1239 Phase B — collapse getConfigDirFromHome chain into getGlobalConfigHomeFragment

AC2 slice 2. Collapses the 14-branch runtime->global-config-home source-fragment
chain in bin/install.js getConfigDirFromHome (the hook path.join() codegen mapping)
into a single getGlobalConfigHomeFragment(runtime) lookup in runtime-name-policy.cts,
sibling to getDirName / getRuntimeLabel.

The antigravity branch stays dynamic in the caller (resolveAntigravityGlobalDir +
path.relative — env-overridable, multi-segment); the prior inner unreachable
`if (!isGlobal) return "'agents'"` (dead: !isGlobal returns at the fn top) is dropped.

Behavior: byte-identical. Fragments preserved verbatim in the table.
- claude/unknown/empty -> default '.claude' fragment
- 14 runtimes (copilot..kimi) -> table lookup
- antigravity -> dynamic (unchanged)

Verification:
- TDD: tests/global-config-home-fragment.test.cjs (golden map + 2 drift guards +
  fallbacks). Red->green.
- 16-runtime golden install parity: byte-identical (hook codegen output unchanged)
- eslint + test-file-count + regression-names clean
- runtime === count in install.js: 115 -> 101 (-14)

* chore(changeset): add Changed fragment for getConfigDirFromHome collapse (#1679)

bin/install.js

@@ -37,7 +37,7 @@ const {
 // installer to the runtime-name-policy leaf (ADR-1508 / #1510 Phase 1) so the
 // conversion module's rewrite engine can consume it without importing
 // bin/install.js. Re-exported below for back-compat consumers/tests.
-const { getDirName, getRuntimeLabel } = require('../gsd-core/bin/lib/runtime-name-policy.cjs');
+const { getDirName, getRuntimeLabel, getGlobalConfigHomeFragment } = require('../gsd-core/bin/lib/runtime-name-policy.cjs');
 const {
   applyWorktreeBaseRef,
   readBaseRefFromSettings,
@@ -510,18 +510,11 @@ function getConfigDirFromHome(runtime, isGlobal) {
     // Local installs use the same dir name pattern
     return `'${getDirName(runtime)}'`;
   }
-  // Global installs - OpenCode uses XDG path structure
-  if (runtime === 'copilot') return "'.copilot'";
-  if (runtime === 'opencode') {
-    // OpenCode: ~/.config/opencode -> '.config', 'opencode'
-    // Return as comma-separated for path.join() replacement
-    return "'.config', 'opencode'";
-  }
-  if (runtime === 'gemini') return "'.gemini'";
-  if (runtime === 'kilo') return "'.config', 'kilo'";
-  if (runtime === 'codex') return "'.codex'";
+  // Global installs. antigravity's home is resolved dynamically (env-overridable,
+  // multi-segment via resolveAntigravityGlobalDir + path.relative) — not a table
+  // entry. (The prior inner `if (!isGlobal) return "'.agents'"` was unreachable:
+  // !isGlobal returns at the top of this function.)
   if (runtime === 'antigravity') {
-    if (!isGlobal) return "'.agents'";
     const antigravityDir = resolveAntigravityGlobalDir();
     const rel = path.relative(os.homedir(), antigravityDir);
     const segments = rel.split(path.sep).filter(Boolean);
@@ -532,16 +525,9 @@ function getConfigDirFromHome(runtime, isGlobal) {
     // stable legacy template so generated path.join() calls remain valid.
     return "'.gemini', 'antigravity'";
   }
-  if (runtime === 'cursor') return "'.cursor'";
-  if (runtime === 'windsurf') return "'.windsurf'";
-  if (runtime === 'augment') return "'.augment'";
-  if (runtime === 'trae') return "'.trae'";
-  if (runtime === 'qwen') return "'.qwen'";
-  if (runtime === 'hermes') return "'.hermes'";
-  if (runtime === 'codebuddy') return "'.codebuddy'";
-  if (runtime === 'cline') return "'.cline'";
-  if (runtime === 'kimi') return "'.config', 'agents'";
-  return "'.claude'";
+  // All other runtimes: single source-of-truth fragment table (ADR-1239 Phase B,
+  // #1679). claude/unknown fall through to the table's default '.claude'.
+  return getGlobalConfigHomeFragment(runtime);
 }
 
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