fix: align repo auto init with GitHub API

[zhangyangyu]

Description

Align repository creation with the GitHub REST API by removing the service-specific add_readme create-repo option, using only auto_init, and seeding an empty README.md when auto-init is requested.

Updates service tests, E2E scripts, smoke coverage, and docs to use the standard field.

Tracking

N/A

Tests

• go test ./internal/gitstore
• go test ./internal/service -run '^$'
• go test ./internal/rest -run '^$'
• go test ./internal/githttp -run '^$'
• go test ./... -run '^$'
• bash -n e2e/code-search-e2e.sh e2e/repo-transfer-lifecycle.sh e2e/vector-search-e2e.sh scripts/backend_smoke.sh
• python3 scripts/doc_lint.py
• git diff --check

Risks / Reviewer Notes

Clients that send the non-standard add_readme field will no longer trigger README seeding; callers should send GitHub-compatible auto_init: true. Unknown JSON fields continue to be ignored by the existing decoder.

SSOT Drift Prevention Checklist

• Checked that this PR does not violate boundaries in docs/architecture.md or docs/module-contracts.md
• If this PR changes package structure or dependencies, updated the relevant SSOT doc
• If this PR changes test conventions or coverage expectations, updated docs/test-strategy.md
• If this PR changes CI, smoke, regression, or workflow expectations, updated docs/ci.md