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>

commands/gsd/from-gsd2.md

@@ -0,0 +1,45 @@
+---
+name: gsd:from-gsd2
+description: Import a GSD-2 (.gsd/) project back to GSD v1 (.planning/) format
+argument-hint: "[--path <dir>] [--force]"
+allowed-tools:
+  - Read
+  - Write
+  - Bash
+type: prompt
+---
+
+<objective>
+Reverse-migrate a GSD-2 project (`.gsd/` directory) back to GSD v1 (`.planning/`) format.
+
+Maps the GSD-2 hierarchy (Milestone → Slice → Task) to the GSD v1 hierarchy (Milestone sections in ROADMAP.md → Phase → Plan), preserving completion state, research files, and summaries.
+</objective>
+
+<process>
+
+1. **Locate the .gsd/ directory** — check the current working directory (or `--path` argument):
+   ```bash
+   node "$HOME/.claude/get-shit-done/bin/gsd-tools.cjs" from-gsd2 --dry-run
+   ```
+   If no `.gsd/` is found, report the error and stop.
+
+2. **Show the dry-run preview** — present the full file list and migration statistics to the user. Ask for confirmation before writing anything.
+
+3. **Run the migration** after confirmation:
+   ```bash
+   node "$HOME/.claude/get-shit-done/bin/gsd-tools.cjs" from-gsd2
+   ```
+   Use `--force` if `.planning/` already exists and the user has confirmed overwrite.
+
+4. **Report the result** — show the `filesWritten` count, `planningDir` path, and the preview summary.
+
+</process>
+
+<notes>
+- The migration is non-destructive: `.gsd/` is never modified or removed.
+- Pass `--path <dir>` to migrate a project at a different path than the current directory.
+- Slices are numbered sequentially across all milestones (M001/S01 → phase 01, M001/S02 → phase 02, M002/S01 → phase 03, etc.).
+- Tasks within each slice become plans (T01 → plan 01, T02 → plan 02, etc.).
+- Completed slices and tasks carry their done state into ROADMAP.md checkboxes and SUMMARY.md files.
+- GSD-2 cost/token ledger, database state, and VS Code extension state cannot be migrated.
+</notes>