feat(mcp): add machine-readable tool errors

[NovusEdge]

MCP tool failures expose a stable error code so callers can handle missing VMs, access refusals, and rate limits without parsing prose. The first text block retains the existing message. A second JSON text block and namespaced metadata carry the same error object.

Existing success schemas and typed handler output remain unchanged. The two new refusal codes are access_denied and rate_limited. Existing JSON-message redaction runs before the message is copied into the new fields.

Validation: separate test-author and implementation commits; expected failures reproduced before implementation; just check, just lint, and just test passed. In-process MCP client tests cover sentinel mapping, refusal messages, JSON escaping and redaction, and success-schema compatibility. Sol reviewed the complete change, followed by one focused correction and final verification. No guest or boot behavior changes.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between bbb62d8 and db792c0.

📒 Files selected for processing (7)

• docs/reference/json.md
• docs/reference/mcp.md
• internal/cli/wire/errors.go
• internal/mcpsrv/access.go
• internal/mcpsrv/error_contract_test.go
• internal/mcpsrv/ratelimit.go
• internal/mcpsrv/server.go

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