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, secrets: an honest result contract, and two leaks closed ...

A pass over the sub-agent system, plus the two findings from the previous one.

Leaks:

- `redact_args` / `is_sensitive_key` (navi/tools/_internal/redact.py) mask
  credential-shaped keys at every log site that prints tool arguments — by key
  name, for all tools, so a future MCP tool is covered without declaring
  anything. ssh_exec passwords were sitting in the prod journal in plaintext.
- `mcp_servers.d/*.json` are tracked in git and carried the scaffolding
  machine's absolute paths, so navi-3d/navi-web silently vanished from the
  toolset off that machine. They now hold project-relative paths, resolved in
  `McpClient.open_transport` — not at load time, because create_mcp_server
  round-trips every config file through save_mcp_servers.

Sub-agent result contract:

- `run_ephemeral` returns a `SubAgentOutcome` (text + status: ok, timeout,
  max_iterations, thinking_stall, user_stop, context_overflow) instead of
  `(text, bool)`. Every exit routes through `SubAgentRunner._finish`, so token
  accounting is never dropped and a partial run always carries a progress
  report — the stop-after-stream path used to return a bare string.
- `spawn_agent` renders its header from the status. Before this, every short
  run reached the parent as "hit iteration limit", and the parent repeats that
  diagnosis to the user — a timeout was reported as an iteration limit.
- `ContextTooLargeError` no longer kills a sub-agent: the parent compresses its
  own context, a sub-agent has no session to compress into, so it stops with
  `context_overflow` and hands back what it did rather than surfacing as
  "Sub-agent failed: …" with the whole run discarded.
- The result is capped at 8000 chars (head and tail kept) so a long run cannot
  flood the parent's context, and tool arguments in the progress report go
  through `redact_args`.
- The navi-3d calling convention left the runner for the server's own tool
  schemas — a generic runner should not know one MCP server's path shapes.
- Validation moved inside spawn_agent's try: a bad argument is a clean
  ToolResult the model can read, not an exception in the parent's loop.

Tests: 1636 passed, 1 skipped. New: SubAgentOutcome paths and headers, result
truncation, context overflow, clean argument failures.

merge

refactor: migrate MCP config to directory-based mcp_servers.d ...

Replaces monolithic mcp_servers.json with per-server files in
mcp_servers.d/<name>.json. Filename stem becomes the server name.

- load_mcp_servers() reads directory globs, auto-migrates legacy file
- save_mcp_servers() writes per-server files, cleans up stale ones
- Update docstrings in manager.py and context_builder.py

Auto-migration: existing servers (gnexus-book, navi-3d, navi-web) plus
new ones (project_health, time_toolkit) now live in mcp_servers.d/.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>