feat(import): add /gsd-from-gsd2 reverse migration from GSD-2 to v1 (#2072)

Adds a new command and CLI subcommand that converts a GSD-2 `.gsd/`
project back to GSD v1 `.planning/` format — the reverse of the forward
migration GSD-2 ships.

Closes #2069

Maps GSD-2's Milestone → Slice → Task hierarchy to v1's flat
Milestone sections → Phase → Plan structure. Slices are numbered
sequentially across all milestones; tasks become numbered plans within
their phase. Completion state, research files, and summaries are
preserved.

New files:
- `get-shit-done/bin/lib/gsd2-import.cjs` — parser, transformer, writer
- `commands/gsd/from-gsd2.md` — slash command definition
- `tests/gsd2-import.test.cjs` — 41 tests, 99.21% statement coverage

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

get-shit-done/bin/gsd-tools.cjs

@@ -154,6 +154,10 @@
  *   learnings copy                       Copy from current project's LEARNINGS.md
  *   learnings prune --older-than <dur>   Remove entries older than duration (e.g. 90d)
  *   learnings delete <id>                Delete a learning by ID
+ *
+ * GSD-2 Migration:
+ *   from-gsd2 [--path <dir>] [--force] [--dry-run]
+ *             Import a GSD-2 (.gsd/) project back to GSD v1 (.planning/) format
  */
 
 const fs = require('fs');
@@ -1070,6 +1074,14 @@ async function runCommand(command, args, cwd, raw, defaultValue) {
       break;
     }
 
+    // ─── GSD-2 Reverse Migration ───────────────────────────────────────────
+
+    case 'from-gsd2': {
+      const gsd2Import = require('./lib/gsd2-import.cjs');
+      gsd2Import.cmdFromGsd2(args.slice(1), cwd, raw);
+      break;
+    }
+
     default:
       error(`Unknown command: ${command}`);
   }