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

SKILLS_INDEX.md

@@ -70,17 +70,21 @@ everything load twice.
 
 ### Crush, Pi, OpenCode — run `install.sh`
 
-These have no plugin system, so they get symlinks — but **one directory link
-each**, never one per skill:
+These have no plugin system, so they get symlinks — but through one managed
+directory, never one link per skill per harness:
 
 ```
-~/.agents/skills           -> $SKILLS_CHECKOUT_DIR/skills   (Pi reads this directly)
+~/.agents/skills/          (real directory; Pi reads it directly)
+~/.agents/skills/tdd       -> $SKILLS_CHECKOUT_DIR/skills/tdd
+~/.agents/skills/xlsx      -> $SKILLS_EXTRA_SOURCES entry
 ~/.config/crush/skills     -> ~/.agents/skills
 ~/.config/opencode/skills  -> ~/.agents/skills
 ```
 
-Adding a skill therefore wires it everywhere with no re-run, and a broken link
-is one broken link instead of 42.
+Total links are N + one per harness. `SKILLS_EXTRA_SOURCES` (colon-separated
+directories) lets a second library — Anthropic's official set, for instance —
+live alongside this one instead of being displaced by it; name collisions are
+logged rather than silently resolved, and the checkout wins (#27).
 
 ```
 curl -fsSL https://git.d-ma.be/mathias/skills/raw/branch/main/install.sh | bash
@@ -94,10 +98,11 @@ absolute link stored there is broken on one side by construction. See #23.
 
 ### Check a host
 
-`task doctor` (or `bin/doctor`) reports dangling links, links resolving into a
-foreign checkout, count mismatches, stray entries, and leftover Claude
-symlinks. It also runs automatically at session start via the plugin, so drift
-announces itself instead of waiting to be found.
+`task doctor` (or `bin/doctor`) reports dangling links, links into a source
+nobody configured, skills in the checkout with no link, stray entries, leftover
+Claude symlinks, and a Claude Code plugin sitting on an older generation than
+the host already has. It also runs automatically at session start via the
+plugin, so drift announces itself instead of waiting to be found.
 
 ## Skill Relationships