feat: add safe update discovery

[zongqichen]

Summary

• add read-only cfs update output for humans and agents
• add bounded, cached interactive update notices outside the CF data path
• document update, privacy, testing, and Agent Skill behavior

Safety

• never checks from the cf shim or named CF invocations
• suppresses passive checks for JSON, CI, help, uninstall, non-TTY, and development builds
• uses a three-second timeout, a 1 MiB response limit, and an owner-only atomic cache
• reports install commands but never replaces the executable

Validation

• make check
• make security
• make release-check
• CFS_REAL_CF=/usr/bin/cf8 make smoke
• make agent-smoke
• live GitHub Releases check from a release-versioned temporary binary
• Claude Code Opus 4.8 review: no blocking findings

Fixes #16