Sync AGENTS.md, CLAUDE.md, and .claude/rules from updated project_rules
templates: workplan-first session protocol, legacy terminology footnote,
and GET /workplans/ examples.
Introduce credential-routing.template for Codex, Claude Code, Grok, and
llm-connect agents. Wire into agents-codex.template and claude-md.template.
Add propagate_credential_routing.py for surgical rollout without clobbering
repo-specific AGENTS.md extensions (REPO-AGENTS-EXTENSIONS marker).