openapi: validate-linked-issue + check-before-start missing from spec (MCP tools + schemas already exist)

[JSONbored]

⚠️ Definition of Done: this issue must be completed in full, in a single PR. Do not split this
work across multiple PRs, and do not defer any Deliverable below to a follow-up issue. A PR that
satisfies only some of the Deliverables, stubs a required test, or leaves a checkbox
partially-done does NOT resolve this issue and will be closed.

⚠️ Required pattern: mirror how GET /v1/repos/{owner}/{repo}/gate-config/effective is documented
in src/openapi/spec.ts — issue #6611 fixed the exact same class of gap for that route.

Context

Two POST /v1/repos/:owner/:repo/* pre-work-check routes in src/api/routes.ts are live and each
already has an MCP tool with its own Zod output shape in src/mcp/server.ts:

• POST /v1/repos/:owner/:repo/validate-linked-issue → loopover_validate_linked_issue
  (outputSchema: validateLinkedIssueOutputSchema)
• POST /v1/repos/:owner/:repo/check-before-start → loopover_check_before_start
  (outputSchema: checkBeforeStartOutputSchema)

Neither route appears in src/openapi/spec.ts — grep for validate-linked-issue and
check-before-start in that file returns nothing.

Requirements

• In src/openapi/schemas.ts, define ValidateLinkedIssueResponseSchema and
  CheckBeforeStartResponseSchema as z.object({...}) schemas, using validateLinkedIssueOutputSchema
  and checkBeforeStartOutputSchema (the raw Zod shapes already validated in src/mcp/server.ts) as
  the field-level source of truth.
• In src/openapi/spec.ts, registry.register both schemas as components, then add registerPath
  entries for both routes, including their request-body schemas (mirror the MCP tools'
  validateLinkedIssueShape/equivalent check-before-start input shape for the request body).
• Schema is the contract — regenerate and commit. Run npm run ui:openapi after the edits and
  commit the regenerated apps/loopover-ui/public/openapi.json in this same PR. CI enforces this via
  npm run ui:openapi:check (part of npm run test:ci).

Deliverables

• ValidateLinkedIssueResponseSchema and CheckBeforeStartResponseSchema added to
  src/openapi/schemas.ts.
• Both routes registered as OpenAPI paths (request + response) in src/openapi/spec.ts.
• apps/loopover-ui/public/openapi.json regenerated via npm run ui:openapi and committed.
• npm run ui:openapi:check passes in CI.

All deliverables are required in this one PR — there is no follow-up issue.

Test Coverage Requirements

99%+ Codecov patch target (codecov/patch, unsharded via npm run test:coverage) on any new schema
helper code. Add a regression test asserting both paths appear in buildOpenApiSpec()'s output with
the correct method and request/response schemas whose keys match each route's MCP tool shapes.

Expected Outcome

Both pre-work-check routes are documented in GET /openapi.json /
apps/loopover-ui/public/openapi.json, matching what their MCP tools already validate.

Links & Resources

• src/api/routes.ts: POST .../validate-linked-issue, POST .../check-before-start
• src/mcp/server.ts: validateLinkedIssueOutputSchema, checkBeforeStartOutputSchema,
  validateLinkedIssueShape
• src/openapi/spec.ts: /v1/repos/{owner}/{repo}/gate-config/effective registration (pattern to
  mirror)
• OpenAPI spec is missing GET /v1/repos/{owner}/{repo}/gate-config/effective (documented sibling: live-gate-thresholds) #6611 — the precedent issue for this exact class of gap

[linear-code]

JSO-6459