ci: add Homebrew tap update to release workflow

Matches the quickbooks-cli release pattern:
- Build artifacts named by target (wpx-{target}.tar.gz)
- After builds, downloads artifacts, computes SHA256 checksums,
  generates Formula, and pushes to osodevops/homebrew-tap
- Install via: brew install osodevops/tap/wordpress-cli

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

refactor: remove MCP server — CLI only

Remove the wpx-mcp crate and all MCP references from code and docs.
This project is a CLI tool, not an MCP server. Removed:
- crates/wpx-mcp/ (entire crate)
- Mcp/McpCommands from cli.rs and main.rs
- MCP sections from README.md and CLAUDE.md
- MCP tool definitions and dispatch logic
- mcp/model-context-protocol GitHub topics

6 crates remain: wpx-cli, wpx-core, wpx-api, wpx-auth, wpx-config, wpx-output.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

fix: resolve all clippy warnings for clean CI

- Collapse if-let into match arm (error.rs)
- Use #[default] derive instead of manual Default impl (format.rs)
- Replace consecutive str::replace with char array pattern (tools.rs)
- Remove redundant variable rebindings (fleet.rs)
- Remove unnecessary borrows (main.rs)

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>