core: anchor restricted-profile boundaries in subagent prompts ...

Sub-agents of non-admin profiles replaced the parent system prompt with
their own worker prompt, so the "one directory, no network, no secrets"
rules never reached them. build_subagent_system_prompt now appends a
[Boundaries] block for every non-admin profile, whatever prompt was
chosen.

Align the profile prompts with the actual tool surface alongside it:
allow 3D sub-agents to draft geometry but not publish or decide, publish
a preview PNG with the STL in both 3D profiles, document the assistant's
remaining tools, canonicalise MCP names, and fix the developer/navi_code
spawn ending, the server_admin field list, the discuss memory_search
guidance, and the navi-3d/navi-web/gnexus-book instructions.

subagents: a worker persona, a live prompt fallback, and guards that scale ...

- assistant and designer_3d get their own subagent_system_prompt.txt; without
  one a sub-agent ran on the orchestrator's prompt, or since ce55325 on nothing
  but the scope anchor, and rarely finished the step it was given
- build the sub-agent system prompt in one place; the documented fallback to
  profile.system_prompt had become dead code
- the thinking-stall guard killed healthy cloud reasoners at 12k chars after
  13-35s; raise the char bound to 48k, keep the 60s wall, and make both settings
- a 300s run cap cut long ops work in half; raise it to 600s, configurable