fix(sync): zero-op sync still opens an anchor-only PR when upstream touched only carved-out paths

[MarkMichaelis]

Problem

A sync whose upstream range touches only consumer-owned / carved-out paths
still produces a commit -- and, on a protected branch, a branch + PR whose entire
diff is a one-line syncedAt bump in .sdlc-ai-sync.json.

This is the same empty-PR complaint as #224. The guard added by #230 only covers
the case where the anchor already equals the upstream head:

$syncStateChanged = ($ops.Count -gt 0) -or ($mergedPaths.Count -gt 0) -or ($anchorSha -ne $upstreamHead)

When the anchor is behind the head but every upstream commit in the range
touched a carved-out path (run.ps1, run.Tests.ps1,
.github/workflows/copilot-setup-steps.yml, .github/agents/tests/), the third
clause is $true, so the state file is rewritten, the tree goes dirty, and an
empty sync commit/PR is manufactured.

Reproduced live in a consuming repo: anchor 42d3b44, upstream head 5264227.
The two intervening upstream PRs (#232, #234) touched only run.ps1 /
run.Tests.ps1, both consumer-owned. Files to update: 0, yet a real run would
open a content-free PR.

Second defect -- dishonest message

Write-SyncPlan prints the zero-op line unconditionally:

Files to update: 0 (already at upstream 5264227 -- nothing to sync)

The consumer is not at 5264227; its anchor is 42d3b44. Two upstream
commits exist, they simply carry no managed-path changes. The message should not
claim an anchor position that is false.

Proposed fix

1. Drop the ($anchorSha -ne $upstreamHead) clause so a zero-op sync never
   rewrites the state file. This is safe: the anchor feeds only
   Test-LocalDriftOnManagedPaths and Get-UpstreamOps, both of which compare
   managed-path content. Zero ops means no managed path differs between anchor
   and head, so an older anchor yields identical results -- it just re-diffs a
   slightly wider (and still cheap) range until real managed content lands.
2. Make the zero-op message distinguish "anchor == head" from "upstream moved but
   nothing managed changed."

Acceptance criteria

• Anchor behind head + zero ops => no commit, no branch, no PR, syncedAt
  untouched.
• Anchor == head + zero ops => unchanged behavior (still no commit).
• Ops > 0 => anchor advances to the upstream head exactly as before.
• The zero-op message does not claim the consumer is at a head it is not at.