feat: add authenticated generic slack api command

[codesoda]

Summary

• Add slack api <method-or-url> for Slack Web API methods without dedicated CLI commands.
• Reuse stored OAuth/browser credentials, workspace selection, and token overrides.
• Support GET/POST, raw and typed fields, JSON parameter files/stdin, and full JSON responses.
• Restrict endpoints to Slack API methods and disable redirects to protect credentials.
• Add bounded rate-limit retries, shared token resolution, and README documentation.

Testing

• 26 dedicated end-to-end tests in tests/cli_api.rs, using a local mock server and fake tokens; no Slack login required.
• cargo build
• SLACK_RUN_MOCK_TESTS=1 cargo test
• cargo test --test cli_api
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --document-private-items

All checks above passed locally. Live Slack integration tests and coverage checks were not run.

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