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

.changeset/tidy-elks-sing.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 2260
+---
+**Headless MemPalace capture no longer fails silently** — the headless invocation `mempalace mine <path> --wing <wing> --room <room>` used a `--room` flag that does not exist on the `mine` subcommand (only `search` accepts `--room`), causing every headless/no-MCP capture run to fail with `unrecognized arguments: --room` and silently skip (onError: skip). The fix replaces the flag with MemPalace's documented room-assignment mechanism: stage the artifact under a room-named subfolder with a `mempalace.yaml` taxonomy so `detect_room()` assigns it via folder-path match. (#2220)