docs: update model selection compatibility - #1521

Addressed both Round 1 findings in commit 27a843d2.

F1 — Keep Gemini and Antigravity as distinct backends

• Restored Gemini as a separate row in docs/slash-commands.md.
• Marked it as a legacy backend instead of implying that it was renamed to Antigravity.
• Clarified the June 18, 2026 transition boundary: Google AI Pro, Ultra, and free-tier individual accounts no longer work with Gemini CLI, while Enterprise and API-key access remain supported.
• Added a migration notice with Google's official announcements to docs/gemini.md.

F2 — Reconcile the OpenCode statement in the Slack guide

• Removed the stale claim that only Kiro exposes model options and that OpenCode does not emit them.
• Linked the Slack guide to the central slash-command compatibility table.
• Kept Slack slash commands unsupported for the two independent routing constraints: thread slash-command restrictions and missing thread context in channel-level payloads.
• Updated the Discord comparison to refer specifically to those two constraints.

Validation

• git diff --check passes.
• Confirmed the internal slash-commands.md#models-and-agents target exists.
• Changes remain documentation-only.
• Reviewed the final diff against F1 and F2.

Please re-review commit 27a843d2.

Addressed the Round 2 authentication finding in commit 6a464e0c.

F1 — Qualify Gemini authentication after the June 18 transition

Updated the docs/gemini.md Authentication section to separate the supported paths by account type:

• API-key authentication remains documented as its own supported path.
• OAuth is now qualified for Enterprise or Google Cloud organizational accounts with supported Gemini Code Assist or Google Cloud access.
• Google AI Pro, Google AI Ultra, and free-tier individual OAuth accounts are explicitly identified as no longer served by Gemini CLI and are directed to Antigravity CLI.

This removes the contradiction between the migration notice and the later authentication instructions.

Validation

• git diff --check passes.
• The fix changes only docs/gemini.md.
• Manually checked the final wording against the June 18 access boundary already cited in the guide.

Please re-review commit 6a464e0c.

What This PR Does

This PR updates /models compatibility documentation for Gemini, Antigravity, OpenCode, and Devin, adds Antigravity model-selection guidance, corrects Devin's command reference, and replaces the stale Slack backend inventory with a link to the central compatibility table.

How It Works

The documentation describes ACP configOptions, session/set_config_option, Antigravity's live model discovery with cache and static fallback, and [pool].default_config_options. It keeps Gemini as a distinct active backend, centralizes Google's June 18 individual-account transition in the Gemini guide, and clearly labels the OpenCode and Devin evidence boundaries.

Findings

Addressing External Reviewer Feedback

Round 1

Restore Gemini as a distinct backend and keep Antigravity as an additional row.

✅ Addressed in 27a843d2: Gemini and Antigravity have separate compatibility rows. The associated review thread is resolved.

Reconcile the OpenCode support claim with the contradictory Slack guide.

✅ Addressed in 27a843d2: the stale Slack claim was removed and replaced with a link to the compatibility table while retaining Slack's independent routing limitations. The associated review thread is resolved.

Round 2

Qualify Gemini OAuth guidance for the post-June 18 individual-account cutoff.

✅ Addressed in 6a464e0c: API-key, supported organizational OAuth, and discontinued individual OAuth paths are separated and backed by the cited maintainer announcements.

Round 3

Separate the Gemini backend lifecycle from individual-account access policy.

✅ Addressed in 5f69640e: the row is named gemini, contains only the capability result, and links to the canonical migration guide. The earlier inline thread is now outdated because its requested line was replaced.

Reconcile the PR Review Contract and Validation sections with the final scope.

✅ Addressed in the current PR body: the goal, non-goals, residual risks, acceptance criteria, validation, five-file scope, and four introduced links now match the reviewed result; the empty Closes # placeholder is gone.

@agent-rapi

Earlier findings were addressed in 27a843d2 and 6a464e0c.

✅ Confirmed: all claimed fixes remain present at the exact reviewed head, and the final Round 3 fixes complete the documented review contract.

Reviewer Coverage

5. Three Reasons We Might Not Need This PR

1. Static compatibility tables drift - upstream ACP clients and vendor policies can change faster than OpenAB documentation releases.
2. Two claims still lack live OpenAB captures - OpenCode and Devin rely on disclosed source or ACP-client evidence pending end-to-end captures.
3. Backend guides already contain partial setup details - users can configure models without a central matrix, although discovery and comparison are harder.