refactor(provider-core): add ProviderHost as the server port for providers

[juliusmarminge]

Drivers reached into the server for its config, settings service, background policy, and attachment store, so no provider could live outside apps/server.

This adds ProviderHost to provider-core, the one service a provider uses to reach the server: paths (cwd, home, state, probe cache), settings reads and change streams, the background-work gate, and attachment path resolution. The server implements it once (ProviderHostLive). The managed status provider and every driver now take ProviderHost instead of BackgroundPolicy. Pi's adapter and driver use it for their cwd, probe cache, and attachments. @t3tools/provider-testing gains a scoped test host.

No behavior change.

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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: ce31ac6 · Source CI: failure

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Superseded by the squashed 3-PR stack #17299 → #17300 → #17302 (same changes, regrouped; Effect service modules renamed to PascalCase).