Support manifest-declared PR policy

[codeforester]

Summary

• Add github.pr manifest schema for default, label-triggered, and path-triggered PR sections.
• Add a base_pr_policy renderer and wire basectl gh pr create to use project policy while preserving Fixes #<issue>.
• Declare Base's own PR policy and update workflow docs plus .ai-context/.

Issue

Fixes #403

Validation

• env PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m unittest cli.python.base_setup.tests.test_manifest
• env PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest cli/python/base_pr_policy/tests/test_engine.py cli/python/base_github_projects/tests/test_engine.py -q
• env BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash bats cli/bash/commands/basectl/tests/gh.bats --filter 'basectl gh pr create'
• rg --files -g '*.py' -0 | xargs -0 /Users/rameshhp/.base.d/base/.venv/bin/python -m pylint --rcfile=.pylintrc
• git diff --check
• git diff --cached --check
• env -u BASE_HOME BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash ./bin/base-test

Docs Impact

• Updated docs/github-workflow.md, docs/command-reference.md, and docs/repo-baseline.md.

AI Context Impact

• Updated .ai-context/COMMANDS.md and .ai-context/WORKFLOWS.md.

Demo Impact

• None. CLI workflow behavior and docs only.