refactor(settings): drop the legacy per-driver providers map

[juliusmarminge]

ServerSettings.providers (one blob per built-in driver) listed every provider's settings schema in contracts, which kept providers from moving into their own packages. providerInstances is now the only provider configuration.

• An unconfigured built-in default slot runs with the driver's default config. Codex and Claude start enabled; the rest start off. The ACP Registry opts out of a default instance through driver metadata.
• Migration: on load, a settings file that still has the old map moves each customized entry into its driver's default slot (an explicit instance wins), applies provider history to the cursor/grok/opencode opt-ins as before, and is rewritten once without the old key. An unreadable file is never rewritten. This matters for real installs: some still point Codex/Claude at custom binaries only through the old map.
• Web settings, onboarding, and model selection read instances only. Resetting a default instance just removes it.

Tests cover the migration (customized entries, explicit-instance precedence, history opt-ins, explicit disables, untouched invalid files) and the new defaults.

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Part of the provider-packages stack: #17282 → #17283 → #17284 → #17285 → #17286 → #17287 → #17288.

Done by Claude Opus 5.5 in Claude Code.

🤖 Generated with Claude Code

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

ℹ️ No successful main baseline artifact is available yet. This run establishes the initial measurement.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	—	5.0 KiB	—	6.8 KiB	✅
Codex	Thread snapshot wire	—	3.8 KiB	—	4.9 KiB	✅
Codex	Live turn WebSocket wire	—	1.2 KiB	—	2.0 KiB	✅
Codex	Live turn WebSocket decoded	—	20.9 KiB	—	29.3 KiB	✅
Codex	Live turn messages	—	2	—	8	✅
Claude	Total thread wire	—	5.0 KiB	—	6.8 KiB	✅
Claude	Thread snapshot wire	—	3.8 KiB	—	4.9 KiB	✅
Claude	Live turn WebSocket wire	—	1.2 KiB	—	2.0 KiB	✅
Claude	Live turn WebSocket decoded	—	21.2 KiB	—	29.3 KiB	✅
Claude	Live turn messages	—	2	—	8	✅

Baseline: unavailable · PR result: 62a9a4a · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 108.5 KiB
• Claude decoded thread snapshot: 108.8 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.

[juliusmarminge]

Superseded by the squashed 3-PR stack #17299 → #17300 → #17302 (same changes, regrouped; Effect service modules renamed to PascalCase).