v0.12.0: Add --config-file flag and secrets-only env var default to c…

…offer run

Add COFFER_ENV environment variable support and validate environment …

…names

Output nested JSON/YAML from resolve command instead of flat env vars

coffer resolve -f json/yaml now preserves the nested config structure
with secrets resolved, making output suitable for SDK consumption.
Dotenv format continues to output flat env var key-value pairs.

Remove macOS Gatekeeper quarantine on cask install

Fix YAML list values resolving to empty strings

Flatten now joins []any slices as comma-separated strings instead of
silently dropping them via the default case.

Add Homebrew tap and install script distribution

Add homebrew_casks config to GoReleaser, POSIX install script with
checksum verification, and expand README installation section with
Homebrew, install script, go install, and manual download options.

Release v0.9.0