docs: make public GCS examples portable

[Pigbibi]

Summary

• replace the deployment-specific execution-report bucket in .env.example with an explicit public placeholder
• replace deployment-specific snapshot bucket examples in the runtime guide while preserving every URI path
• add focused regression coverage for public GCS example portability and URI syntax

Scope

Documentation and public example configuration only. No workflow, OIDC, verifier, runtime, permission, dependency, deployment, or live-operation changes.

Validation

• uv run --no-sync pytest -q tests/test_public_config_examples.py — 1 passed
• uv run --no-sync ruff check --exclude external . — passed
• uv run --no-sync python -m compileall -q application entrypoints notifications scripts tests *.py — passed
• focused scope/content identity check — passed
• git diff --check — passed
• uv run --no-sync pytest -q --tb=no — 28 failed, 249 passed; the same 28 failures were present on the clean origin/main baseline before this change

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[chatgpt-codex-connector]

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