feat: install command auto-installs latest GitHub release

[hellodk]

Problem

scripts/champ install builds the VSIX from the current checkout and installs it. Following the docs (README.md:90) or the release notes means running a manual version-pinned curl:

curl -L "https://github.com/hellodk/champ/releases/latest/download/champ-2.0.1.vsix" -o /tmp/champ.vsix && code --install-extension /tmp/champ.vsix --force

This is fragile: the /latest/download/<asset> name must be hand-typed, stale CDN caches serve 404s, and code --install-extension output is polluted by VS Code-internal url.parse() DEP0169 deprecation noise (warning is emitted by the VS Code CLI itself, not this repo).

Acceptance criteria

• scripts/champ install (no args) installs the latest GitHub release of hellodk/champ — no version knowledge required
• scripts/champ install build installs the locally-built VSIX (old behavior)
• No [DEP0169] DeprecationWarning noise in install output (suppressed, since VS Code emits it internally)
• Interactive menu updated to match both modes
• Header usage comment in scripts/champ updated

Tests required

• Manual test: scripts/champ install — installs the latest release from GitHub, exit 0, no DEP0169 noise
• Manual test: scripts/champ install build — builds current checkout, exit 0
• Manual test: scripts/champ menu options 2 / 9 route correctly