Update adapters.md: adapter authoring smoke validation has shipped

[justinmclean]

Summary

Rewrite the Known gaps bullet in tools/spec-loop/specs/adapters.md that says
adapter authoring smoke validation is missing. The check exists.

Background

The bullet reads "Adapter authoring smoke validation is missing. The docs define
the expected README contract, but no validator currently checks that each adapter
declares capability, prerequisites, privacy/credential handling, operations, and
config keys." All five of those are now checked by validate_adapter_authoring.

Spec Known gaps sections are not just documentation: the spec-loop plan beat
reads them to choose the next work item, so a gap that describes already-shipped
code sends the loop at finished work.

Where to look

• tools/spec-loop/specs/adapters.md line 165 - the bullet to rewrite.
• tools/skill-and-tool-validator/src/skill_and_tool_validator/__init__.py -
  validate_adapter_authoring is called at line 3710. The regexes it uses for
  credentials, operations and config keys are defined near the top of the file.

Verify by finding the call site, not the definition.

Acceptance criteria

• The bullet describes the shipped check instead of claiming it is missing.
• The description says which of the five declarations are checked and that
  it applies to contract:* adapters only, not substrate:* tools.
• uv run --project tools/spec-validator spec-validate tools/spec-loop/specs/
  passes.

Estimated effort

~ 30 minutes for someone new to the codebase.

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