validation: reject unsafe slugs in 'socrates init <slug>'

[CryptoJones]

Path(base) / args.project had three foot-guns when args.project
was untrusted:

1. Absolute slug: Path("base") / "/etc/passwd" returns "/etc/passwd"
   in Python pathlib. socrates init /etc/passwd --base ~/foo would
   try to scaffold under /etc/passwd, not under ~/foo. Even though
   the user owns the CLI invocation, this violates the mental model
   ("init creates a project named UNDER --base").

2. Path-separator slug: socrates init a/b quietly creates nested
   structure; socrates init ../tmp/bad escapes --base.

3. Empty slug: socrates init "" resolves to the base dir itself,
   risking damage to existing siblings.

Added a _validate_slug() helper that rejects:

• empty / whitespace-only
• containing '/' or '\' (single path component only)
• absolute paths
• '..' or any '..' segment
• '.' or '..' literal
• NUL bytes (defensive)

Accepts alphanumerics, dash, underscore, dot — covers normal slugs
like quarterly-rebates, v0.8.0, .hidden.

Tests (new tests/test_init_slug_safety.py, 22 tests):

• 11 parametrized rejection cases
• 7 parametrized acceptance cases (normal slugs unchanged)
• 4 end-to-end CLI integration tests proving:
  • absolute slug exits 2, /etc/passwd/docs/ never created
  • traversal slug exits 2, sibling dir never created
  • nested slug exits 2 with helpful message
  • empty slug exits 2 with helpful message

169/169 tests pass; ruff + mypy clean.

Self-review caveat: doesn't reject Windows reserved names (CON, NUL, PRN, AUX, COM1-9, LPT1-9) or trailing dots/spaces (Windows strips silently). Unix-only safety; Windows operators can still trip themselves up.