secrets: a placeholder may carry a fallback, and web search stops being a Settings field ...

An unset ${SEARXNG_URL} failed the whole navi-web connection, all three of its
tools and not only search, because resolve_secrets raises on any placeholder it
cannot fill. The rule is right for credentials, where a blank value quietly
becomes anonymous access, and wrong for a setting, where an empty SearXNG URL
costs one backend rather than the server. ${NAME:-fallback} (POSIX-ish, empty
fallback allowed) says which is which, and writing the fallback down makes it a
decision in the config instead of an accident of a variable nobody set.

Settings.brave_search_api_key and .searxng_url were read by nothing — the
navi-web server has its own config — so they go, with docs/config.md pointing at
mcp_servers.d/navi-web.json and deploy/env.template naming SEARXNG_URL where a
deploy actually sets it.

The literal-secret guard still holds: a placeholder name is a credential unless
it is in SETTINGS_VARIABLES, and a settings placeholder must sit in an env block.

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.