security(content): block .env secret reads

[thehoff]

Summary

• add a shared sensitive .env path guard with documented template exceptions
• block native content readers and proxied content-reader invocations before file contents are read
• document the proxy override and add regression coverage for read, grep, proxy grep, and template files

Tests

• cargo test --test system_hardening dotenv -- --nocapture
• cargo test sensitive_paths -- --nocapture
• cargo test --test system_hardening
• cargo test --test proxy_nudge

Issue/repo note: issue rtk-ai#206 uses contextcrawler proxy .env shorthand, but the reproduced bypass is contextcrawler proxy grep PASSWORD .env; the implementation targets proxied content-reader invocations.