Closes the remaining atoms of #112600.
A) The CLI startup path already passes `-m` as target_model (c358a6fba0), but
its siblings still resolved credentials against config's `default`: the CLI
auth-fallback rung, `--resume` credential re-resolution, the gateway
provider-override helper (channel overrides, persisted /model switches,
API-server provider refresh), the gateway fallback chain, the TUI /model
switch-from runtime and ACP agent construction. With a `*-free` default the
OpenCode free-tier rung fired first and a Go-only model was built against
the keyless Zen relay ("Model mimo-v2.5 is not supported"). Each now passes
the effective model; `_resolve_runtime_agent_kwargs_for_provider` grows an
optional `target_model` and the two test stubs of it accept the kwarg.
B) normalize_opencode_base_url rewrote the /zen vs /zen/go segment for ANY
provider matched by opencode_provider_family, including custom providers
merely named after a family (`opencode-go-bridge`, #85589) whose relay the
user declared explicitly in `providers:`. The family heal now applies to the
built-in canonical providers only; custom prefix-named providers keep their
per-model api_mode routing and /v1 handling. Documented in the providers
guide.
C) Same function: the official-host check uses parsed.hostname (a port no
longer defeats the heal) and only the path is edited, so query/fragment
round-trip instead of being dropped.
Fixes#112600
931b5ff9e7