fix(#2220): replace invalid mempalace mine --room with detect_room() staging (#2260)

* fix(#2220): replace invalid 'mine --room' with detect_room() staging approach

mempalace mine has no --room flag (only search does) — verified against
MemPalace 3.5.0 official docs (mempalaceofficial.com/reference/cli.html).
The headless capture path used --room, causing every headless/no-MCP run
to fail with 'unrecognized arguments: --room' and silently skip capture.

Fix: replace the flag with a staging-based approach that uses detect_room()'s
documented folder-path match — stage the artifact under a room-named subfolder
with a mempalace.yaml room taxonomy, then run 'mempalace mine <stage> --wing'.

Docs sources cited in-file:
- CLI reference:  https://mempalaceofficial.com/reference/cli.html
- Mining guide:   https://mempalaceofficial.com/guide/mining.html
- Config guide:   https://mempalaceofficial.com/guide/configuration.html

Changes:
- skills/gsd-mempalace-capture/SKILL.md: headless staging instructions
- commands/gsd/mempalace-capture.md: same
- capabilities/mempalace/fragments/capture-problems.md: reference staging
- .gitignore: exclude .planning/.mempalace-stage/
- tests/mempalace-capture-headless-invocation.test.cjs: regression test
- Golden install parity fixtures + workflow-size baseline regenerated

* docs: backfill changeset PR number (#2260)

* fix: regenerate golden fixtures after next merge

capabilities/mempalace/fragments/capture-problems.md

@@ -13,7 +13,7 @@ For each confirmed bug/issue resolved in this wave:
 
 1. **Resolve the wing** (`mempalace.wing`, else `project_code`, else project dir) and target `room: problems`.
 2. **Dedupe first.** Call `mempalace_check_duplicate` (interactive) before filing so re-runs don't create duplicate drawers.
-3. **File the drawer verbatim.** Store the problem statement and its fix as a drawer in `room: problems` — interactive: `mempalace_add_drawer`; headless: `mempalace mine` / `mempalace hook run`. Include provenance (`source_file`, phase id).
+3. **File the drawer verbatim.** Store the problem statement and its fix as a drawer in `room: problems` — interactive: `mempalace_add_drawer`; headless: stage the artifact under the `problems/` folder and run `mempalace mine` (no `--room` flag — see [CLI reference](https://mempalaceofficial.com/reference/cli.html); room assignment is via `detect_room()` folder-path match per the [mining guide](https://mempalaceofficial.com/guide/mining.html); use the same staging pattern documented in `gsd-mempalace-capture` Step 3). Include provenance (`source_file`, phase id).
 4. **Mirror the KG fact** when `mempalace.mirror_kg` is on: add `(<bug>, fixed_by, <fix>)` with `valid_from` = the phase date via `mempalace_kg_add`.
 5. **Mode awareness** (`mempalace.memory_mode`). Under `augment` the fact is an *additive* mirror alongside `.planning/graphs/`. Under `kg_backend`/`replace` the palace is the *authoritative* store for the fact; GSD still writes `.planning/graphs/` through its normal graphify, so an unreachable palace never loses it.