mcp: credentials leave the tree, and sub-agents lose the task board ...

Five tracked configs (gnexus-creds, gntodo, hard-panel, synapse, tgclient)
carried live bearer tokens — in git, in every clone, and handed to the admin
client by GET /admin/mcp/config. They now name a variable, and the value lives
in the service .env (chmod 600, untracked).

Substitution happens once, at transport open, next to the project-relative
path resolution: that is the single place a transport is built, so every
stored and serialised config stays placeholder-only and save_mcp_servers —
reached by create_mcp_server and PUT /admin/mcp/config — cannot write a secret
back into a tracked file. A variable that is not set refuses the connection
instead of dropping the header, which would fail open: a server may answer an
unauthenticated request as an anonymous user rather than returning 401.

Sub-agents also drop gntodo, gnexus-creds, tgclient and synapse from their
scopes, and the runner injects a scope anchor unconditionally — a sub-agent
asked to echo one word had picked a project off the task board and researched
it for forty iterations.

Docs: docs/mcp.md#secrets, a migration recipe in deploy/UPDATE.md, the variable
names in .env.example and deploy/env.template, and a regression guard over the
tracked configs.

The old values stay in git history; rotating all five at their sources is the
remediation, not a follow-up.

config: add the gntodo MCP server