Part of #1195 — Phase 1

Integrate ContentSanitizer into ContextBuilder so all untrusted data is automatically sanitized before entering agent context.

Crates: zeph-core
Depends on: SEC-1.2, SEC-1.3

Tasks:

• Add ContentSanitizer to ContextBuilder (initialized from config)
• All tool results pass through sanitizer with appropriate ContentSource
• Spotlighting delimiters injected around untrusted blocks in system prompt
• tracing::debug! when injection patterns are flagged (structured: source, pattern, location)
• Integration tests: mock tool returning injection payload, verify wrapping in context

Files: crates/zeph-core/src/context.rs, crates/zeph-core/src/agent.rs