feat(install): serve several sources instead of displacing them

~/.agents/skills was a single link straight at this checkout. That encoded an
assumption nobody had stated — that this checkout is the only source of skills
on the host. On flamingo it was not: Crush's directory held Anthropic's
official set, and the only way to install this library was to move that set
aside. A capability loss dressed up as a tidy-up.

The canonical location becomes a real directory of per-skill symlinks, fed by
the checkout plus anything in SKILLS_EXTRA_SOURCES. Harnesses keep their one
directory link, so the count is N + one per harness rather than N x harnesses,
which is what #23 was actually about.

The three requirements #27 named, each with a test:
- collisions are logged with both paths and the checkout wins, never resolved
  in silence;
- removing a skill from a source prunes its link;
- anything in the managed directory that is not our symlink is reported and
  left in place — the installer does not delete content it did not create.

doctor learns the same model: sources are recorded at ~/.agents/skills-sources,
a link into a configured source is healthy, a link into an unknown one is not,
and the old count check becomes a per-skill check because a per-skill layout
can lose one skill without changing any total. The pre-#27 wiring is named as
legacy rather than reported as damage. count_skills needed find -L; without it
symlinked skill directories count as zero.

Verified live on koala: migrated in place to 42 per-skill links, then a second
source added a 43rd alongside them, then a re-run without that source pruned it
back to 42. Only doctor finding throughout was the genuine stale plugin.

Closes #27
Bump-Type: minor

README.md

@@ -40,16 +40,42 @@ moves — CI bumps it on every release.
 curl -fsSL https://git.d-ma.be/mathias/skills/raw/branch/main/install.sh | bash
 ```
 
-Idempotent. It clones or fast-forwards the checkout, then creates **three**
-symlinks — one directory link per harness, never one per skill:
+Idempotent. It clones or fast-forwards the checkout, then populates one
+**managed directory** and gives each harness a single link to it:
 
-| Link | Target |
+| Path | Target |
 |---|---|
-| `~/.agents/skills` | `$SKILLS_CHECKOUT_DIR/skills` (Pi reads this path directly) |
+| `~/.agents/skills/` | a real directory, one symlink per skill (Pi reads this path directly) |
+| `~/.agents/skills/<skill>` | `$SKILLS_CHECKOUT_DIR/skills/<skill>`, or another configured source |
 | `~/.config/crush/skills` | `~/.agents/skills` |
 | `~/.config/opencode/skills` | `~/.agents/skills` |
 
-Adding a skill wires it everywhere with no re-run.
+Total links are N + one per harness — not N x harnesses, which is what #23 was
+actually about.
+
+### More than one source of skills
+
+`~/.agents/skills` used to be a single link straight at this checkout. That
+encoded an assumption nobody had stated — that this checkout is the only source
+of skills on the host — and on flamingo the only way to install this library
+was to displace Anthropic's official set, which lived in Crush's directory.
+Real capability loss dressed up as a tidy-up (#27).
+
+Point `SKILLS_EXTRA_SOURCES` at any number of extra directories, colon
+separated, each holding `<skill>/SKILL.md`:
+
+```
+SKILLS_EXTRA_SOURCES=~/.config/crush/skills.anthropic bash install.sh
+```
+
+- The checkout is always first and always wins a name collision — but the
+  collision is **logged**, never resolved in silence. Quietly picking one is
+  how stale content wins.
+- Removing a skill from a source removes its link on the next run.
+- Anything in the managed directory that is not a symlink is reported and left
+  alone. install.sh does not delete content it did not create.
+- The source list is recorded at `~/.agents/skills-sources` so `bin/doctor`
+  can tell a link into a known source from a link into a forgotten one.
 
 ### Where the checkout must live
 
@@ -74,6 +100,7 @@ default to point into `~/dev`.
 | `AGENTS_SKILLS_DIR` | `$HOME/.agents/skills` |
 | `CRUSH_SKILLS_DIR` | `$HOME/.config/crush/skills` |
 | `OPENCODE_SKILLS_DIR` | `$HOME/.config/opencode/skills` |
+| `SKILLS_EXTRA_SOURCES` | unset (colon-separated extra source directories) |
 
 Antigravity and Mistral Vibe targets were removed — unused.