openapi: /v1/opportunities/find + /v1/issue-rag/retrieve 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

src/api/routes.ts defines two discovery-oriented POST routes via named path constants, each backed
by an MCP tool with its own Zod output shape already defined in src/mcp/server.ts:

• POST /v1/opportunities/find (OPPORTUNITIES_FIND_PATH) → loopover_find_opportunities
  (outputSchema: findOpportunitiesOutputSchema)
• POST /v1/issue-rag/retrieve (ISSUE_RAG_RETRIEVE_PATH) → loopover_retrieve_issue_context
  (outputSchema: issueRagOutputSchema)

Neither route appears in src/openapi/spec.ts — grep for opportunities/find and issue-rag in
that file returns nothing.

Requirements

• In src/openapi/schemas.ts, define FindOpportunitiesResponseSchema and
  IssueRagRetrieveResponseSchema as z.object({...}) schemas, using findOpportunitiesOutputSchema
  and issueRagOutputSchema (the raw Zod shapes already validated in src/mcp/server.ts) as the
  field-level source of truth. Add matching request-body schemas mirroring the two tools'
  inputSchema/shape constants.
• In src/openapi/spec.ts, registry.register both schemas as components, then add registerPath
  entries for both routes.
• 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

• FindOpportunitiesResponseSchema and IssueRagRetrieveResponseSchema (plus matching
  request-body schemas) added to src/openapi/schemas.ts.
• Both routes registered as OpenAPI paths 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
response schemas whose keys match findOpportunitiesOutputSchema/issueRagOutputSchema.

Expected Outcome

Both discovery 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: OPPORTUNITIES_FIND_PATH / ISSUE_RAG_RETRIEVE_PATH route registrations
• src/mcp/server.ts: findOpportunitiesOutputSchema, issueRagOutputSchema
• 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-6465