[DPE-10234] docs: add AGENTS.md and CLAUDE.md with comprehensive agent guidelines

[marceloneppel]

Issue

Coding agents working on this repository make architectural mistakes (violating charm patterns, Patroni coordination, relation interfaces) and produce code that fails CI (linting, type checking, test failures) due to lack of comprehensive guidance.

Solution

Add a comprehensive AGENTS.md file covering:

• Architectural rules: module responsibilities, Patroni-only lifecycle management, relation handler patterns, leader-only writes, peer data access, event deferral, status setting, rolling restarts, retry patterns, Juju secrets for sensitive data, vendored libraries in lib/.
• Code quality rules: copyright headers, style (ruff), type checking (ty).
• Testing rules: unit test patterns, integration test setup (jubilant preferred for new tests), conftest auto-mocking, cloud credentials only for backup/restore tests.
• Workflow checklist: step-by-step CI validation procedure, all constants in constants.py.

Add a CLAUDE.md symlink pointing to AGENTS.md so that Claude Code auto-loads the same guidelines at session start.

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.