[codex] prepare flavor 0.3.1

[PerishCode]

Why

The public install surface still had separate install and uninstall entrypoints, while install and upgrade were effectively the same operation. Beta releases also carried channel-specific manager script publishing even though the management logic is channel-agnostic.

What

• replaces root install/uninstall scripts with a single manage.sh and manage.ps1 entrypoint
• keeps install as the only install/update action and uninstall as the removal action
• adds explicit --retain / --retain=true / --retain=false handling, with non-interactive installs preserving old versions by default
• publishes manage.sh and manage.ps1 only at the public root during stable releases
• points release metadata at manage.* and removes install/uninstall metadata fields
• updates release smoke tests, Cloudflare redirect helpers, init checks, README, and AGENTS
• bumps flavor-cli to v0.3.1

Compatibility

This is an intentional hard cut: install.sh, install.ps1, uninstall.sh, and uninstall.ps1 are removed from the repository and no longer emitted by release metadata. New public usage is manage.sh / manage.ps1.

Tests

• cargo fmt --all --check
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo test --locked --workspace
• cargo run --locked -p flavor-cli -- check --root . --config flavor.json
• python3 scripts/dev/antlr.py check
• python3 scripts/init.py
• sh -n manage.sh and release shell scripts
• python3 -m py_compile for updated Python support scripts
• local manage.sh smoke with --retain=false pruning and uninstall
• local manage.sh non-interactive default retain smoke