feat: add shared mirror cache for repeated repository clones

[skulidropek]

Summary

• adds a shared git mirror cache under ~/.docker-git/.cache/git-mirrors for clone acceleration across containers
• uses git clone --reference-if-able ... --dissociate when a mirror exists
• bootstraps mirror from the freshly cloned repo when cache is missing
• logs cache reuse explicitly ([clone-cache] using mirror: ...) for runtime verification
• skips .docker-git in project config traversal to avoid scanning cache/state directories
• updates managed state .gitignore to ignore .cache/git-mirrors/
• adds a dedicated CI e2e scenario scripts/e2e/clone-cache.sh

Proof

Tests

• pnpm --filter @effect-template/lib test -- tests/usecases/docker-git-config-search.test.ts tests/usecases/state-repo-gitignore.test.ts
• pnpm --filter @effect-template/docker-git test -- tests/core/templates.test.ts
• bash scripts/e2e/clone-cache.sh (executed in CI job E2E (Clone cache))

E2E cache assertions

clone-cache.sh performs two independent docker-git clone runs for the same repo URL and checks:

• first run emits [clone-cache] mirror created: ...
• mirror directory exists in host state .docker-git/.cache/git-mirrors/<hash>.git
• second run emits [clone-cache] using mirror: ...

Closes#48