fix(core): reduce snapshot repository setup

[opencode-agent]

Summary

• replace eight git config subprocesses with an OpenCode-owned include file and one canonical include appended directly to the snapshot repository config
• preserve Git-generated and template-provided settings, including multiple existing include.path values
• keep repeated repository creation idempotent without parsing unrelated Git configuration
• leave all later Git command construction unchanged

Results

• focused snapshot test process executions: 38 → 30 (strace, including the test runner)
• focused snapshot path launches zero git config subprocesses
• focused snapshot test: 30/30 repeated runs
• Git tree integration test: 5/5 repeated runs

Checks

• bun test test/git.test.ts test/snapshot.test.ts
• bun typecheck
• git diff --check

Requested by: @rekram1-node (Aiden Cline via Slack)