Add encrypted Agent defaults and frozen Session execution overrides

[SaladDay]

Saved Agents can now store editable model, harness and encrypted model-provider defaults. Session creation resolves explicit overrides, then saved defaults, then deployment defaults, and atomically freezes the effective configuration and private credentials. Agent edits/deletion and operator configuration changes do not alter existing Sessions; idempotent retries recover the original Session without replay.

Provider overrides are complete bundles, so a new endpoint never receives an inherited key. Public responses expose only safe configuration and api_key_configured. The existing hosted-only credential boundary remains in place. Includes migration 000059, generated OpenAPI/SQL, TypeScript contracts/examples and architecture documentation.

Validation:

• Contract/API/server, full PostgreSQL store/execution, encryption, sqlc drift, executor/harness and client/Web unit checks passed.
• Isolated real-model tests passed for Codex Responses, Claude SDK Anthropic and MiniMax Code Anthropic: inheritance, overrides, Agent edit/delete, idempotency, Core restart and microsandbox suspend/resume with retained files. Deployment fallback snapshots also survive operator endpoint/key/native-option changes.
• Independent blind review completed; the identified deployment-native-options preservation issue was corrected and a fresh review found no actionable issues.
• make check was run: 123 browser cases passed and four unrelated Dashboard cases failed. All four failures reproduce on pristine baseline d15da0d; the full gate is therefore not green. An initial isolated qualification Core connection closed once; recovery failed the interrupted Turn without replay, and subsequent real-model runs passed without recurrence. No production deployment was changed.
• Final merged-main integration passed: 382 client tests and 715 Web unit tests, contracts/API/server, PostgreSQL public-defaults/Subagent checks and generation drift checks. The final binary also completed one real continuation in each retained harness Session (exactly 3→4 completed Turns, original history/configuration unchanged). Twenty native Turns completed successfully after the initial interrupted attempt.

Compatibility: newly frozen hosted deployment providers now use the same HTTPS validation as explicit providers; legacy HTTP endpoints must be changed to HTTPS. Native provider options remain privately encrypted and preserved.

Task: AGENT-EXEC-DEFAULTS-001 — https://vrfi1sk8a0.feishu.cn/wiki/EA3CwVSRviqe1XkZ0tAcq7FqnFc#doxcn8404vBIN5js7B06m1gZZwd