Add missing native procedure imports across packages

After the native relocation change, native procedures require explicit imports. This commit adds the missing imports identified through static analysis:

• sigil-changes: Add (sigil process) for exit, (sigil io) for file-exists?
• sigil-repl: Add (sigil process) for exit, (sigil math) for bitwise ops
• sigil-websocket: Add (sigil io) and (sigil math) for encoding/bitwise
• sigil-mcp: Add (sigil process) for exit
• sigil-nrepl: Add (sigil string) for string-trim
• sigil-docs: Add (sigil process) for exit, (sigil io) for read
• sigil-jwt: Add (sigil io) and (sigil math) for utf8/floor
• sigil-test: Add (sigil process) for exit

Also includes: - scripts/find-missing-imports.sgl: Static analysis tool for finding missing native imports by cross-referencing C registrations with .sgl file usage - test/compiler/test-deps.sgl: Update to use current keyword-based package API instead of deprecated s-expression syntax