Skills 3/6: skills you turn on are offered to the model, and every pull shows #1405

Step 3 of #1277. It lands in the single pull request for #1277 (branch feat/1277-skills).

What

• Where the choice is kept. Three places hold the same two lists, on: [names] and off: [names]. The narrowest scope that names a skill wins; otherwise the skill is off. Every write goes through one engine door.

• Catalog. Skills that are on are listed in message[0] (name, one-line description, size), beside the standing orders and folders. The catalog changes only when a setting changes or a folder is attached.
• The skill tool. Its name is constrained to the skills that are on. It returns the body wrapped as <skill name="…" dir="…">…</skill>, with the skill's bundled files listed. Relative paths resolve against dir, and the skill's folder is allow-listed for read.
• Absent, not broken. With no skill on, neither the catalog nor the tool exists.
• Every pull is visible. Each call draws its own transcript row naming the skill and its size, for example used skill pdf-processing · 3.1 KB. It is never folded into prose.
• Loaded once. A second pull of the same skill in one conversation returns a short "already loaded". Compaction keeps a loaded skill.
• allowed-tools pre-approves nothing.
• Words. prompts/system.md says what a skill is and when to open one. docs/CHAT-V3.md's "dropped skills & rules" line is invalidated by this pull request's change entry.

Acceptance, end to end

Real binary with a stub model:

• With pdf-processing turned on globally, the request's system prompt lists it, and the tools include skill with pdf-processing in its enum.
• When the stub calls skill with pdf-processing, the transcript draws used skill pdf-processing · followed by its size. The tool result carries the body and the skill's dir.
• With nothing on, the request has no skill tool and no catalog section.
• A project skills.json turning the skill off overrides the global on. A conversation turning it on overrides the project off.
• The belt gate names skill. The manual answers "which skills can you use on your own?".