fix: Implement proper sandbox enforcement with corrected Landlock

Based on security findings from @aecs4u (PR #21), with critical Landlock fix.

## Security Issues Fixed

1. **PathValidation was no-op** — now properly validates against deny lists
2. **Background commands bypassed sandbox** — now wrapped before spawning
3. **Landlock semantics were inverted** — CRITICAL: original code would ALLOW
   credentials instead of denying them

## Landlock Fix (the critical change)

The original implementation misunderstood Landlock's security model. Landlock
is ALLOWLIST-based: you specify what IS allowed, everything else is denied.

The fix:
- Allow system paths: /usr, /lib, /bin, /etc, /proc, /sys, /dev (read)
- Allow temp paths: /tmp, /var/tmp (read+write)
- Allow workspace (full access)
- Credentials denied BY OMISSION (not in allowlist = denied)

## Changes (cherry-picked from PR #21, sandbox commits only)

- src/sandbox.rs: Complete sandbox implementation with corrected Landlock
- docs/SANDBOX.md: Comprehensive documentation
- tests/sandbox_enforcement.rs: Integration tests

## Attribution

Security finding and initial implementation by @aecs4u.
Landlock semantics fix reviewed against Landlock API docs.

Closes #22
Supersedes #21

Author: rexlunae

Cargo.toml

@@ -108,6 +108,12 @@ base64 = "0.22"
 # HTTP date parsing (for Set-Cookie expires)
 httpdate = "1.0"
 
+# Terminal styling
+colored = "3.1"
+indicatif = "0.18"
+unicode-width = "0.2"
+rpassword = "7"
+
 # Matrix messenger support (optional)
 # Features: e2e-encryption (default), sqlite (persistence), rustls-tls (TLS)
 # rustls-tls is required when default-features = false
@@ -126,11 +132,9 @@ chromiumoxide = { version = "0.8", default-features = false, features = ["tokio-
 [target.'cfg(unix)'.dependencies]
 libc = "0.2"
 
-# Terminal styling
-colored = "3.1"
-indicatif = "0.18"
-unicode-width = "0.2"
-rpassword = "7"
+# Landlock sandbox (Linux 5.13+ kernel)
+[target.'cfg(target_os = "linux")'.dependencies]
+landlock = "0.4"
 
 # Patches for crypto compatibility
 [patch.crates-io]