fix(scenarios): assertScenarioLocalBranchInputSafe is a dead safety guard -- never called in production

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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

src/scenarios/input-model.ts:189-220's exported assertScenarioLocalBranchInputSafe is framed (by its own comments and the recent #8328 fix, which added the "reject non-array changedFiles" branch) as the guard preventing source-content upload in scenario inputs -- it scans for FORBIDDEN_SOURCE_UPLOAD_KEYS (sourceContent, diff, patch, etc.) and oversized changedFiles entries. Grep across src/, packages/, and scripts/ shows zero production importers -- only its own unit test (test/unit/scenario-input-model.test.ts) calls it. The real production caller, packages/loopover-mcp/lib/local-branch.ts:125, calls a different, weaker local function assertSourceUploadDisabled() (lines 707-711) that only checks the LOOPOVER_UPLOAD_SOURCE env flag -- it does not scan for forbidden keys or oversized content at all, even though CollectLocalBranchMetadataInput (same file, lines 44-56) passes several unknown-typed fields (scenarioNotes, validation, projectedCredibility, branchEligibility, labels) straight through to the output payload.

Requirements

Wire assertScenarioLocalBranchInputSafe into the real local-branch collection path in packages/loopover-mcp/lib/local-branch.ts (either as a replacement for, or in addition to, assertSourceUploadDisabled), so the forbidden-key/oversized-content scan actually runs on the production entry point. Do not simply delete the function -- its scanning logic is the documented safety mechanism and has no replacement; wiring it in is the correct fix.

Deliverables

• packages/loopover-mcp/lib/local-branch.ts's production local-branch collection path calls assertScenarioLocalBranchInputSafe (in addition to or in place of assertSourceUploadDisabled)
• A regression test asserting the actual production entry point (local-branch.ts) rejects a forbidden-key/oversized payload, not just the unit-level assertScenarioLocalBranchInputSafe test

All of the above Deliverables are required in the same PR unless the deliverable text itself states otherwise.

Test Coverage Requirements

src/** -- 99%+ patch coverage, branch-counted, on the new wiring and its rejection path.

Expected Outcome

The production local-branch scenario-collection path actually enforces the forbidden-source-upload-key and oversized-content scan, closing a dead-security-guard gap.

Links & Resources

• src/scenarios/input-model.ts:189-220
• packages/loopover-mcp/lib/local-branch.ts:44-56,125,707-711
• test/unit/scenario-input-model.test.ts

[linear-code]

JSO-6163