refactor(packaging): rely on gateway runtime defaults

[drew]

Summary

Remove package-time gateway TOML generation for Homebrew, Debian, RPM, and Snap. Package services now start openshell-gateway directly, keep OPENSHELL_* launch overrides working when present, and rely on runtime defaults plus an optional gateway TOML.

Related Issue

N/A

Changes

• Added gateway runtime defaults for the optional XDG TOML path, SQLite state path, and local generated TLS bundle.
• Updated Homebrew to read $XDG_CONFIG_HOME/openshell/gateway.toml when present and fall back to an existing Homebrew prefix config only when the XDG config is absent.
• Updated Debian/RPM services to generate certs with openshell-gateway generate-certs and launch the gateway without --config or --db-url defaults.
• Updated Snap to avoid writing TOML, set Snap-specific DB/TLS defaults through env, and read $SNAP_COMMON/gateway.toml only when it exists.
• Removed the shared config generator and RPM PKI shell helper.
• Kept env-variable compatibility without documenting the old env file name.
• Updated package docs, man pages, gateway config/auth references, and release formula tests.

Testing

• mise run pre-commit
• uv run pytest python/openshell/release_formula_test.py
• cargo test -p openshell-driver-docker
• cargo test -p openshell-driver-podman grpc_endpoint
• cargo test -p openshell-server cli::tests
• cargo test -p openshell-server defaults::tests
• cargo test -p openshell-core paths::tests
• cargo test -p openshell-core config::tests
• E2E tests added/updated (not applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)

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[github-actions]

Label test:e2e applied for b07f372. Open the existing run and click Re-run all jobs to execute with the label set. The E2E Gate check on this PR will flip green automatically once the run finishes.

[TaylorMutch]

/ok to test b07f372