fix(ci): harden downstream bump inputs

[zhanghanduo]

Summary

• keep repository_dispatch and workflow_dispatch values out of generated shell source
• validate release tags with a strict allow-list before writing GITHUB_ENV
• repeat validation inside the repin helper for defense in depth
• cover valid PEP 440 release forms plus shell, newline, whitespace, and path injection inputs

Why

The downstream template is the authoritative source for ApodexHarness PR #524. Direct GitHub-expression interpolation let a crafted manual-dispatch value alter the shell program before the old v-prefixed check ran; the same loose check also allowed GITHUB_ENV injection and invalid branch names.

Verification

• uv run pytest -q tests/test_downstream_repin.py (14 passed)
• uv run ruff check agent_core tests scripts .github/downstream/repin_agent_core.py
• python3 -m py_compile .github/downstream/repin_agent_core.py tests/test_downstream_repin.py
• git diff --check