engine(intake): `validateIdeaSubmission` rejects the `IdeaTarget` object shape it returns

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⚠️ 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.

Context

packages/loopover-engine/src/idea-intake.ts exports the discriminated union added by #7635:

export type IdeaTarget =
  | { kind: "existing"; repo: string }
  | { kind: "provision" };

IdeaSubmission.targetRepo is typed as IdeaTarget (line 32), and validateIdeaSubmission returns
{ ok: true, idea: IdeaSubmission } whose targetRepo is a resolved IdeaTarget (line 130).

But the validator's own targetRepo branch (lines 103-108) only recognises two input shapes:

• a bare "owner/name" string (the back-compat wire form), which it resolves to
  { kind: "existing", repo };
• an object whose kind is exactly "provision".

Anything else falls through to else errors.push("target_repo_required"). So passing the canonical
{ kind: "existing", repo: "acme/widgets" } — the exact shape the function itself produces, and the
shape any TypeScript caller writing against the exported IdeaSubmission type will construct — is
rejected with target_repo_required.

The round-trip fails: validateIdeaSubmission(validateIdeaSubmission(raw).idea) returns
{ ok: false, errors: ["target_repo_required"] } for every valid existing-repo submission. The
provision variant round-trips correctly, so this is an asymmetry between the two arms of one union,
not a deliberate wire-format restriction.

This is reachable from real callers: src/mcp/server.ts:3731 and src/mcp/server.ts:3747
(loopover_intake_idea / loopover_plan_idea_claims) and src/api/routes.ts both call
validateIdeaSubmission and then thread validated.idea.targetRepo — an IdeaTarget — into
buildClaimPlan, which already accepts both variants (line 285). Only the validator lags.

Requirements

• validateIdeaSubmission MUST accept { kind: "existing", repo: <string> } as a targetRepo input
  and resolve it to that same IdeaTarget, applying the identical repo format check the bare-string
  branch applies (currently the regex on line 104) and pushing the identical target_repo_malformed
  error code when the repo value fails it.
• An { kind: "existing" } object whose repo is absent, not a string, or an empty/whitespace-only
  string MUST push target_repo_malformed (not target_repo_required) — it is a recognised-but-invalid
  target, matching how the bare-string branch reports a recognised-but-malformed slug.
• An object whose kind is neither "existing" nor "provision" MUST keep pushing
  target_repo_required, unchanged.
• The bare-string and { kind: "provision" } behaviours MUST be byte-for-byte unchanged.
• No new exported symbol, no new error code beyond the two that already exist.

⚠️ Required pattern: mirror the existing targetRepo branch structure in
validateIdeaSubmission (packages/loopover-engine/src/idea-intake.ts:103-108) — one added
else if arm on the same if/else if/else chain, reusing the same regex constant and the same two
existing error codes. What does NOT satisfy this issue: introducing a separate exported
normalizeIdeaTarget() helper as a second parallel validation surface; loosening the targetRepo
check to accept any object with a repo property; changing the exported IdeaTarget union; or
adding a third error code.

Deliverables

• validateIdeaSubmission({ id, title, body, targetRepo: { kind: "existing", repo: "acme/widgets" } })
  in packages/loopover-engine/src/idea-intake.ts returns { ok: true } with
  idea.targetRepo deep-equal to { kind: "existing", repo: "acme/widgets" }, asserted by a new
  named case in test/unit/idea-intake-bridge.test.ts.
• A named regression test asserting the round-trip: feed a validated result's own idea object
  back into validateIdeaSubmission and assert ok === true and that targetRepo is unchanged.
• { kind: "existing", repo: "not-a-slug" } returns { ok: false, errors: ["target_repo_malformed"] },
  asserted by a new test case.
• { kind: "existing" } (no repo) and { kind: "existing", repo: "" } each return
  { ok: false, errors: ["target_repo_malformed"] }, asserted by new test cases.
• { kind: "bogus" } still returns { ok: false, errors: ["target_repo_required"] }, asserted by
  a new test case.
• Existing bare-string and { kind: "provision" } cases still pass unmodified.

All Deliverables above are required in a single PR. A PR that satisfies only some of them — for
example adding the { kind: "existing" } accept arm without the two invalid-repo cases, or without
the round-trip regression test — does not resolve this issue.

Test Coverage Requirements

packages/loopover-engine/src/**/*.ts is inside coverage.include in vitest.config.ts (second
entry, right after src/**/*.ts) and carries its own engine Codecov flag. The 99%+ branch-counted
codecov/patch gate applies here exactly as it does to src/**. Every changed conditional needs both
arms exercised: the new kind === "existing" arm must be tested with a valid repo, a malformed
repo, a missing repo, and a non-string repo; the fall-through else must still be tested with an
unrecognised kind. The round-trip test above is the required named regression test for this fix.

Expected Outcome

IdeaSubmission values built against the exported IdeaTarget type validate successfully instead of
failing with target_repo_required, and validateIdeaSubmission is idempotent over its own output for
both union variants rather than only the provision one.

Links & Resources

• packages/loopover-engine/src/idea-intake.ts (lines 22-36, 90-136, 281-297)
• src/mcp/server.ts:3731, src/mcp/server.ts:3747 (the two MCP tool call sites)
• src/api/routes.ts (POST /v1/loop/intake-idea, POST /v1/loop/plan-idea-claims)
• Closed Add existing-vs-provision target discriminated union to IdeaSubmission.targetRepo #7635 introduced the IdeaTarget union this validator only half-implements.

[linear-code]

JSO-6683