Declare each Harness's support once

[SaladDay]

Each Harness now declares what it supports in one place, and one validator checks every selection against that declaration.

• One declaration. Each internal/harnessconfig/<kind> returns a proto.Declaration next to its Configuration(): the capabilities, message and image limits, MCP origins, reserved labels and patterns, output-schema rules, and Conflicts. Each daemon adapter starts from it and can only clear support.
• The heartbeat only narrows. Core closes a connection whose heartbeat widens the static declaration or names an unknown kind. Fields that never narrow at runtime are not sent.
• One validator. proto.ValidateSelection is the only check of a declared rule.
  • Core runs it when an Agent with a saved Harness is created or updated (new), at Session creation, at input and function-result admission, at device selection and at the final claim.
  • The Runtime runs it once, when dispatch admits an execution_prepare, before any Executor factory.
  • Errors. A rejection is 400 unsupported_or_invalid_configuration, with the configuration path as param.
• Deleted.
  • services/core/internal/engine/, including the generated constructor map and the catalog profile;
  • the execution policy callbacks;
  • the adapter and bridge re-checks;
  • the capabilities that never narrowed;
  • ExecutionControls.WebSearch with WebSearchControl;
  • the hand-written harness-capabilities.md. The coverage ledger now lists each Harness's differences.
• Docs. harness-onboarding.md drops step 4 and gains "Declare support". runtime-protocol.md covers the heartbeat. zh translations updated.

Public behavior changes

• Three rejections move from invalid_request to unsupported_or_invalid_configuration, the code whitespace-only text already used: unsupported message images, Claude function-result images that are failed, remote or not inline PNG/JPEG, and MiniMax Code function results. Each is a Harness limitation, not a malformed request.
• Claude now treats a function-parameter root that has no type, or a union that includes object, as an object. A union root used to be rejected.

Checks

• Go:
  • Focused tests on 21 packages, including TestCoreErrorCatalog.
  • Integration tests.
  • -race -count=50 on the changed concurrent tests.
  • darwin and windows builds.
• Generated outputs: make check-harness-catalog; make openapi produces no diff.
• Web and docs:
  • pnpm test in claude-sdk-adapter.
  • The website build and tests.
  • make check-names check-docs check-ci.

No live Harness qualification ran on this PR. It runs once after R6, because R6 changes the same adapters. A shared blind review covers this PR together with R6 and PR5b.

Net change: -1,498 lines.

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