docs(readme): note that pre-#120 .gitignore entries are safe to delete

[MarkMichaelis]

Background

PR #120 stopped propagating Samples/HAR-Original/ and .dogfood-output/ to consumer .gitignore files (the former is now owned by the api-wrapper-scaffold skill; the latter is upstream-only). However, the .gitignore merge in Merge-FileFromUpstream is union-only -- it never removes entries. Consumers that synced before PR #120 still have both lines in their .gitignore.

The entries are inert (the directories don't exist in typical consumers), but they are noise that a maintainer reviewing their .gitignore may stumble over.

Proposed change

Add a one-paragraph note to README.md under the bootstrap / sync section explaining:

• These two entries propagated to consumers before PR feat(sync): upstream-only gitignore markers + .gitattributes.template bootstrap #120.
• They are safe to delete by hand if not needed.
• Samples/HAR-Original/ should be kept (or re-added by the api-wrapper-scaffold skill) only by consumers that actually use the API-wrapper scaffold.
• .dogfood-output/ is upstream-only and never needs to be in a consumer .gitignore.

Acceptance

• README section exists and clearly lists the two legacy entries with a one-line rationale each.

Out of scope

• Automating removal in Pull-SDLC.ai.ps1 (would require a "deprecated entries" mechanism in the merge engine -- high complexity, low value).

Context

Spawned from PR #120 follow-up review.