fix: recover Core synchronous Git reconciliation without blind retry

[James3014]

This fixes a Core-bound git_commit failure mode found during the Nexus Runtime Wave B1 managed replay. A deterministic pre-effect rejection such as an incomplete Candidate path set could previously admit the SYNCHRONOUS_GIT writer first, leave the session in OUTCOME_UNKNOWN, and permanently wedge later valid commits.

The change moves physical scope/deletion/path-set preconditions ahead of synchronous Git admission. For already-stranded sessions it reuses the existing physical reconciliation path, clears only the exact unresolved synchronous writer, and returns CORE_MUTATION_RECONCILED_RETRY_REQUIRED; the recovery call never automatically retries or creates a commit.

The production change is limited to src/server.ts, with regression coverage in src/server.test.ts. Independent verification against exact head 07b274cdff1982d704d1b09da103885bbb7b191b passed the five hostile wedge cases, full server/Core/Git Candidate suites, typecheck, diff-check, and the full repository npm test (exit 0; one pre-existing environment-conditional macOS /var alias skip was observed in the full suite).