fix: enforce immutable session configuration laws

[loganj]

🤖

Summary

Defines and enforces three architectural laws for session configuration:

• a session's harness, provider, and model form a compatible configuration;
• each session has exactly one effective configuration; and
• Berd displays the configuration the session uses.

Provider/model inventory is provenance-aware: runtime and cached candidates remain advisory, while successful ACP inventory responses establish the models Berd may synthesize, persist, repair, prepare, or send. Existing explicit intent survives unavailable proof; authoritative empty or excluding proof clears unsupported selections.

The same authority governs persisted persona migration, default and new-session selection, model pickers, session preparation, managed-provider repair, and final ACP wire behavior. External agent harnesses remain distinct from Goose model providers, and prompt sends enforce provider policy synchronously without performing inventory I/O.

Compatibility

Rebased onto the standard session/delete lifecycle and queued-message work now on main. Session-target preparation preserves the active dispatch lease and mutation intent while model changes are deferred or superseded.

Related issue

Supersedes squareup/berd#1114 after the repository migration. Builds on the provider-selection behavior introduced by squareup/berd#1085.

Testing

• Full Vitest suite: 540 files passed; 6,350 tests passed; 1 skipped
• just check
• just tauri-check
• just clippy
• git diff --check block/main...HEAD

just ci reached the release-script suite on Linux; its macOS-only ditto packaging test cannot execute there. The remaining frontend and Rust gates above passed independently.