Add read-only basectl release assistant

[codeforester]

Summary

• Add typed optional release: metadata to base_manifest.yaml, including GitHub release data and optional Homebrew handoff fields.
• Add read-only basectl release check|plan|notes with release readiness checks, changelog note extraction, GitHub target output, and Homebrew handoff planning.
• Wire Bash dispatch, shell completions, Base manifest metadata, docs, changelog, and implementation plan coverage.

Validation

• BASE_HOME="$PWD" PYTHONPATH="$PWD/lib/python:$PWD/cli/python" /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest cli/python/base_setup/tests/test_manifest.py -q
• BASE_HOME="$PWD" PYTHONPATH="$PWD/lib/python:$PWD/cli/python" /Users/rameshhp/.base.d/base/.venv/bin/python -m pytest cli/python/base_release/tests/test_engine.py -q
• BASE_HOME="$PWD" bats cli/bash/commands/basectl/tests/release.bats
• BASE_HOME="$PWD" bats cli/bash/commands/basectl/tests/help.bats cli/bash/commands/basectl/tests/completions.bats
• BASE_CACHE_DIR=/private/tmp/base-release-smoke-cache BASE_HOME="$PWD" bin/basectl release plan --version 0.3.0 --manifest base_manifest.yaml
• BASE_CACHE_DIR=/private/tmp/base-release-smoke-cache BASE_HOME="$PWD" bin/basectl release notes --version 0.3.0 --manifest base_manifest.yaml
• git diff --check
• env -u BASE_HOME ./bin/base-test

Demo Impact

• basectl release plan --version 0.3.0 --manifest base_manifest.yaml now prints Base release metadata and the Homebrew tap handoff checklist without publishing anything.

Closes #541
Closes #542
Part of #540
Part of #544