refactor(bin): make caveman hand-invoked only

Sets `disable-model-invocation: true`, so the host withholds caveman from
the model-visible skill listing. It stops paying preload tax and can only
be reached by typing /caveman.

The reason is not the 287 characters. caveman's description was almost
entirely a trigger list -- "caveman mode", "talk like caveman", "less
tokens", "be brief" -- and "be brief" is a false-positive magnet: a user
asking for brevity wants short answers, not dropped articles and
`Respond terse like smart caveman`. Removing the skill from the router
deletes the bad trigger. User's call, made explicitly.

Safe to flag because the flag also hard-blocks the Skill tool, so any
inbound route from another skill would break. Re-checked all 39
descriptions and bodies after four waves of retrofit rewrites: every
mention of caveman is documentation, research notes or the lockfile.
Nothing routes to it.

The description is rewritten as human-facing text, since it is no longer
a routing surface -- it now says how to turn the mode on and off rather
than listing phrases for a reader that can no longer see it.

The two remaining SUGGESTIONs are #108: the boundary-clause check fires on
hand-invoked skills, which ADR-0020 contractually exempts. Advisory, and
not to be "fixed" in the skill.

Preload tax 10,002 -> 9,732 chars across 37 model-visible skills.

Refs #99

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MWb5RQgCL1ye7cGp2RPb2u

plugins/bin/.apm/skills/caveman/SKILL.md

@@ -1,10 +1,11 @@
 ---
 name: caveman
+disable-model-invocation: true
 description: >
-  Ultra-compressed communication mode. Cuts token usage ~75% by dropping
-  filler, articles, and pleasantries while keeping full technical accuracy.
-  Use when user says "caveman mode", "talk like caveman", "use caveman",
-  "less tokens", "be brief", or invokes /caveman.
+  Ultra-compressed output mode: drops articles, filler and pleasantries while
+  keeping technical substance exact. Cuts token usage by roughly 75%. Hand-invoked
+  only — type /caveman to turn it on, "stop caveman" or "normal mode" to turn it
+  off. Stays active across turns until you do.
 ---
 
 Respond terse like smart caveman. All technical substance stay. Only fluff die.

plugins/bin/skills/caveman/SKILL.md

@@ -1,10 +1,11 @@
 ---
 name: caveman
+disable-model-invocation: true
 description: >
-  Ultra-compressed communication mode. Cuts token usage ~75% by dropping
-  filler, articles, and pleasantries while keeping full technical accuracy.
-  Use when user says "caveman mode", "talk like caveman", "use caveman",
-  "less tokens", "be brief", or invokes /caveman.
+  Ultra-compressed output mode: drops articles, filler and pleasantries while
+  keeping technical substance exact. Cuts token usage by roughly 75%. Hand-invoked
+  only — type /caveman to turn it on, "stop caveman" or "normal mode" to turn it
+  off. Stays active across turns until you do.
 ---
 
 Respond terse like smart caveman. All technical substance stay. Only fluff die.