feat: support Google BYOK providers through Pi

[SaladDay]

Change

Allow manually configured google-generative-ai providers in the CLI and TUI, including OpenCode Go Gemini 4. Model requests and connection probes use the existing Pi Google adapter. Pi also owns Google thinking parameters; OpenCode Go's current /inference/go endpoints receive conversation headers.

Production code grows by 63 net lines. No vendored Pi changes, new dependencies, native model discovery, or structured-output expansion. Includes a CLI setup example.

Validation

• All applicable verification gates exercised on macOS / Node 22.23.2 at 863f1a6. The default-concurrency capability run hit an existing updater test's 5-second timeout; rerunning the declared suite with two workers passed 218 files / 5502 tests, with 17 existing skips. Remaining gates passed individually.
• The initial Homebrew Node 26 run hit that updater fixture's copied-Node dynamic-library failure. No unrelated tests or timeouts were changed.
• Built CLI: live OpenCode Go Gemini 4 connection test and file-tool round trip passed using isolated synthetic input.
• Independent blind review passed. Windows/Linux validation remains to CI.
• Performance: basic automatic suite; no transport, history, or tool-loop implementation changes.

Publication and contribution checks

• Contribution uses the existing applicable licenses; existing notices are preserved.
• No credentials, account data, real user content, internal history, or private review material.
• Content-only changes to existing source/test files; no inventory or test-suite registration changes required.
• Usage documentation updated; offline tests and live acceptance are distinguished above.

Maintainer handoff

No publication-scope or license changes. Shared-source port: pending.

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