install.sh: drop .skill-source marker in deployed skill dir

The skill directory at ~/.agents/skills/opencode-mempalace-bridge/ is a
real dir containing a single SKILL.md symlink back into this repo — the
'colocated skill' pattern. Sibling reconcilers (skillset's
deploy-skills.sh, cli_utils's agents-sync.zsh) already handle external
dirs correctly via their existing 'leave real dirs alone' policies, but
a machine-readable marker makes ownership explicit:

  # skill-source: mempalace-toolkit
  # repo: <absolute path>
  # url: ssh://git@gitea.jordbo.se:2222/joakimp/mempalace-toolkit.git

The marker is the convention for any external repo that wants to ship a
colocated skill. The name is generic (.skill-source, not
.managed-by-mempalace-toolkit) so a second colocated skill from a
different repo can reuse the same file name; the first line identifies
the owner.

--uninstall now also removes the marker (only if it still says
mempalace-toolkit) and the now-empty skill dir.

AGENTS.md + README.md describe the pattern and point at sibling docs in
cli_utils/AGENTS-SYNC.md and skillset/README.md that mirror the
convention.

README.md

@@ -142,6 +142,8 @@ The skill is the *short-form checklist* for agents — when to use which wrapper
 
 The skill pairs with the consumer-side [`mempalace` skill](https://github.com/MemPalace/mempalace) — that one covers using the palace (search, diary, KG); this one covers feeding it.
 
+**Colocated skill pattern.** The skill lives here (not in [`skillset`](https://gitea.jordbo.se/joakimp/skillset)) because it moves in lockstep with the wrappers it documents. `install.sh` drops a `.skill-source` marker file in the deployed skill directory so sibling tooling (skillset's `deploy-skills.sh`, cli_utils's `agents-sync.zsh`) can tell the directory is externally owned. See [`AGENTS.md`](AGENTS.md) for the full convention and how to adopt it for future colocated skills.
+
 ---
 
 ## See also