Skills 6/6: skills inside tasks

[AbirAbbas]

Step 6 of #1277. It lands in the single pull request for #1277 (branch feat/1277-skills).

What

• A task node renders its own system prompt, so it gets the same catalog and skill tool for its scope.
• propose_task gains an optional skills list that pins named skills into a node's brief, so the chat can hand skilled work to a task with the skill attached.
• A worker on the bash belt gets the catalog and reads SKILL.md with the shell it has.
• A skill pulled inside a task draws the same row on the task page, among that node's steps.

Acceptance, end to end

With a stub model:

• propose_task with skills: ["pdf-processing"] produces a node whose first request carries that skill's body.
• A node with skills turned on in its scope has the skill tool.
• The task page shows used skill pdf-processing · followed by its size among the node's steps.
• The bash-belt worker's page lists the catalog with each SKILL.md path.
• The propose_task schema and the manual describe skills with the same wording.