docs: Tool-result-only Responses input shipped in v1.12.0; the docs note still says main after v1.11.0 - #178

Reviewed SMG source: dcc5cb0b8f2a64694938c10f02c6caaa0c40d5ef.

Single sentence-level edit serving exactly the scoped concern (release anchor for tool-result-only continuation acceptance). Verified in the pinned v1.12.0 checkout (HEAD dcc5cb0b): validate_responses_cross_parameters (crates/protocols/src/responses.rs:3489-3520) lacks the input_missing_user_message check and contains commit 538ec73e's exact replacement comment, proving the acceptance shipped in v1.12.0; the error string 'Input items must contain at least one message' is absent repo-wide. The similar 'Request must contain at least one message' check in the regular gRPC conversions path (conversions.rs:244-246) counts Tool messages produced from function_call_output items (conversions.rs:150-152), so it does not contradict the claim. Retained claims verified: Harmony 'No function call found for call_id' at harmony/builder.rs:846; the quoted v1.11.0 rejection matches the removed code in the source patch (readable historical evidence, previously documented under the v1.11.0 pin by merged #102). Placement: canonical page corrected in place, release note kept as its own short paragraph, the smg#2723 code-change reference removed from prose, no new page, 7 changed lines. Consistency: responses.md:73 table row already agrees; the adjacent post-v1.12.0 validation note is untouched; greps show no other docs page covers this behavior. Limitations: git ancestry could not be traversed with read-only tools (release inclusion established via checkout contents), and the v1.12.0 boundary assumes no intermediate v1.11.x patch release shipped the change. No feedback threads exist on this PR.