docs: update remaining gpt-5-mini → gpt-5 examples across docs

[aheritier]

Documentation updates

This PR completes the documentation sync for PR #2997 and PR #3012, updating the remaining gpt-5-mini → gpt-5 and gemini-2.5-flash → gemini-3.5-flash references in generic how-to examples across the docs.

Source PRs

• #2997 — updated OpenAI default from gpt-5-mini to gpt-5, Google default from gemini-2.5-flash to gemini-3.5-flash
• #3012 — partial doc sync (covered configuration/overview and configuration/models)

Commits

Commit 1 — gpt-5-mini → gpt-5 in generic how-to examples:

• docs/configuration/hcl/index.md
• docs/configuration/agents/index.md
• docs/configuration/models/index.md
• docs/concepts/multi-agent/index.md
• docs/concepts/models/index.md
• docs/concepts/agents/index.md
• docs/concepts/distribution/index.md
• docs/features/mcp-mode/index.md
• docs/features/cli/index.md
• docs/getting-started/introduction/index.md
• docs/getting-started/quickstart/index.md
• docs/guides/secrets/index.md
• docs/tools/mcp/index.md
• docs/community/troubleshooting/index.md
• docs/providers/overview/index.md

Commit 2 — gemini-2.5-flash → gemini-3.5-flash in generic how-to examples:

• docs/configuration/agents/index.md
• docs/concepts/agents/index.md
• docs/configuration/models/index.md
• docs/guides/go-sdk/index.md
• docs/concepts/models/index.md

Commit 3 — revert contrast-example model names back to gpt-5-mini:

• docs/concepts/multi-agent/index.md (Multi-Model Teams fast model)
• docs/features/mcp-mode/index.md (designer agent alongside claude-sonnet-4-5 engineer)

Not changed

Provider-specific pages (docs/providers/google/, docs/providers/openai/), examples specifically demonstrating model parameters (guides/thinking/), model-picker tool docs, routing cheap-fallback contrast examples (docs/configuration/routing/index.md), docs/providers/custom/index.md, and within-file contrast/tiering examples (docs/concepts/multi-agent/index.md Multi-Model Teams section, docs/features/mcp-mode/index.md designer/engineer example) are intentionally unchanged.

PR #3007 review

PR #3007 (fix: preserve agent field during command expansion) — bug fix only. The docs already correctly document the intended behavior of the agent: field in commands. No documentation update needed.

[docker-agent]

Assessment: 🟢 APPROVE

Reviewed 16 documentation files (442 diff lines). The PR consistently and correctly updates all gpt-5-mini → gpt-5 model name references across the documentation. All changed lines use the correct format (openai/gpt-5, gpt-5, or GPT-5 as appropriate to context).

Scope: Documentation-only changes — no code, schema, or config logic modified.

Findings summary: No confirmed or likely bugs found in the changed code. Two low-severity observations (not in changed lines):

• In docs/concepts/agents/index.md and docs/configuration/agents/index.md, fallback model list examples show the updated openai/gpt-5 alongside unchanged google/gemini-2.5-flash context lines. If the Gemini model rename (gemini-2.5-flash → gemini-3.5-flash) was part of a previous PR (#2997), these context lines may warrant a follow-up update — but they are outside the scope of this PR's changed lines and do not affect correctness of the current changes.

The PR description accurately reflects all changed and intentionally-unchanged files.