Reduce to the declarative provider, as the upstream review asked

[Lookoff-AIMLAPI]

Summary

Reduces this fork's main to what the upstream review asked for:

could you make it just use the api key one, that would be simpler and cleaner I think for this. Declarative only - should be a much smaller change

Upstream PR aaif-goose/goose#11758 is open from this fork's main, so merging this updates that PR in place.

Effect on the upstream PR

	before	after
files	13	4
lines	+801	+75

What remains:

File
crates/goose-providers/src/declarative/definitions/aimlapi.json	the provider definition
crates/goose-providers/src/declarative.rs	one line in expose_declarative_providers!
crates/goose/src/providers/init.rs	registry wiring test
documentation/docs/getting-started/providers.md	one docs row

Authentication is a plain AIMLAPI_API_KEY. The headers block follows the convention the other routers here already use — http-referer and x-title identifying goose — plus the two headers AI/ML API reads to attribute traffic.

Removed

The browser sign-in flow and everything that reached it: the signup_aimlapi module with its loopback server and HTML templates, the example, the config module wiring, and the setup-menu entry with its handler. The setup menu therefore returns to its original ordering and labels.

The flow is not lost — it is kept on pkce/agent-signin (e55acdbd), where it stays available to this fork.

Also trimmed

The registry wiring test additionally asserted where AI/ML API lands in the sorted provider list. That is not what a wiring test is for, it is brittle against any future provider sorting nearby, and asserting our own placement in someone else's suite is not ours to do. What is left mirrors test_gondola_provider_registry_wiring exactly.

Verification

• cargo build -p goose-cli — succeeds. (--no-default-features minus local-inference, which needs libclang for bindgen and is unrelated to this change.)
• cargo test -p goose --lib providers::init — 10 passed, including test_aimlapi_provider_registry_wiring.
• cargo fmt --check — clean.
• Live: goose run on this build answers on aimlapi openai/gpt-5-5 with only AIMLAPI_API_KEY set — no sign-in flow involved.
• All 8 bundled model ids were checked against the live catalog; none are stale.