fix: run the CLI when reached through the bin symlink

[blaipr]

What this changes

cdmon and cdmon-mcp now run when invoked through the symlinks npm link / npm install -g create. The main-module guard resolves both import.meta.url and process.argv[1] through the filesystem before comparing them, via a new isInvokedDirectly() exported for testing. docs/installation.md mentions npm link.

Why

Node resolves the entry point's symlinks before loading it, so import.meta.url names the real dist/cli.js, while process.argv[1] is the path as typed — .../node_modules/.bin/cdmon. The two never matched from an installed command, so the module loaded, ran nothing, and exited 0. cdmon --help printed nothing; a deploy step in CI would have reported success without doing anything. Reproduced with a symlink to dist/cli.js before the change, and confirmed fixed after.

Checklist

• npm test passes
• npm run typecheck passes
• Documentation in docs/ updated if behaviour changed
• CHANGELOG.md updated if the change is user-observable
• No credentials, hostnames or database names in the diff