Part of #1195 — Phase 2

Apply sanitization at ToolExecutor boundary. All tool outputs must carry TrustLevel metadata.

Crates: zeph-tools, zeph-core
Depends on: SEC-1.4

Tasks:

• ToolResult gains trust_level: TrustLevel field
• ShellExecutor: results tagged LocalUntrusted
• WebScrapeExecutor: results tagged ExternalUntrusted, extra sanitization (HTML tag stripping)
• CompositeExecutor: propagates trust level from inner executors
• Audit log: tracing::info! for each tool result with trust level and size
• Unit tests per executor type

Files: crates/zeph-tools/src/lib.rs, crates/zeph-tools/src/shell.rs, crates/zeph-tools/src/web_scrape.rs, crates/zeph-tools/src/composite.rs