fix(security): harden outbound fetch policy

[chaliy]

What

Add a hardened outbound policy profile for fetchkit and document how to deploy it safely in cluster/data-plane environments.

Why

fetchkit should reject more unsafe outbound paths itself, not rely only on cluster networking. Ambient proxy env, internal DNS suffixes, nonstandard ports, and cross-host redirects all widened the fetch surface for shared deployments.

How

• add builder options for ambient proxy suppression, host blocking, port allow-listing, and same-host redirect restrictions
• add ToolBuilder::hardened() and expose it in CLI, MCP, and Python bindings
• enforce the new checks before connect and on every redirect hop
• add SSRF/security regression tests and CLI flag coverage
• add user-facing hardening guidance and update specs/threat model/changelog

Risk

• Medium
• Hardened mode is intentionally stricter and may block deployments that depend on nonstandard ports, internal-looking public DNS names, cross-host redirects, or inherited proxy env

Checklist

• Unit tests are passed
• Smoke tests are passed
• Documentation is updated
• Specs are up to date and not in conflict
• Security review completed for the changed fetch path