docs: document 9 previously undocumented features

New documentation for features that existed in code but had no docs:

New page:
- context-references.md: Full docs for @-syntax inline context
  injection (@file:, @folder:, @diff, @staged, @git:, @url:) with
  line ranges, CLI autocomplete, size limits, sensitive path blocking,
  and error handling

configuration.md additions:
- Environment variable substitution: ${VAR_NAME} syntax in config.yaml
  with expansion, fallback, and multi-reference support
- Gateway streaming: Progressive token delivery on messaging platforms
  via message editing (StreamingConfig: enabled, transport, edit_interval,
  buffer_threshold, cursor) with platform support matrix
- Web search backends: Three providers (Firecrawl, Parallel, Tavily)
  with web.backend config key, capability matrix, auto-detection from
  API keys, self-hosted Firecrawl, and Parallel search modes

security.md additions:
- SSRF protection: Always-on URL validation blocking private networks,
  loopback, link-local, CGNAT, cloud metadata hostnames, with
  fail-closed DNS and redirect chain re-validation
- Tirith pre-exec security scanning: Content-level command scanning
  for homograph URLs, pipe-to-interpreter, terminal injection with
  auto-install, SHA-256/cosign verification, config options, and
  fail-open/fail-closed modes

sessions.md addition:
- Auto-generated session titles: Background LLM-powered title
  generation after first exchange

creating-skills.md additions:
- Conditional skill activation: requires_toolsets, requires_tools,
  fallback_for_toolsets, fallback_for_tools frontmatter fields with
  matching logic and use cases
- Environment variable requirements: required_environment_variables
  frontmatter for automatic env passthrough to sandboxed execution,
  plus terminal.env_passthrough user config

website/docs/user-guide/security.md

@@ -358,6 +358,42 @@ When a blocked URL is requested, the tool returns an error explaining the domain
 
 See [Website Blocklist](/docs/user-guide/configuration#website-blocklist) in the configuration guide for full details.
 
+### SSRF Protection
+
+All URL-capable tools (web search, web extract, vision, browser) validate URLs before fetching them to prevent Server-Side Request Forgery (SSRF) attacks. Blocked addresses include:
+
+- **Private networks** (RFC 1918): `10.0.0.0/8`, `172.16.0.0/12`, `192.168.0.0/16`
+- **Loopback**: `127.0.0.0/8`, `::1`
+- **Link-local**: `169.254.0.0/16` (includes cloud metadata at `169.254.169.254`)
+- **CGNAT / shared address space** (RFC 6598): `100.64.0.0/10` (Tailscale, WireGuard VPNs)
+- **Cloud metadata hostnames**: `metadata.google.internal`, `metadata.goog`
+- **Reserved, multicast, and unspecified addresses**
+
+SSRF protection is always active and cannot be disabled. DNS failures are treated as blocked (fail-closed). Redirect chains are re-validated at each hop to prevent redirect-based bypasses.
+
+### Tirith Pre-Exec Security Scanning
+
+Hermes integrates [tirith](https://github.com/sheeki03/tirith) for content-level command scanning before execution. Tirith detects threats that pattern matching alone misses:
+
+- Homograph URL spoofing (internationalized domain attacks)
+- Pipe-to-interpreter patterns (`curl | bash`, `wget | sh`)
+- Terminal injection attacks
+
+Tirith auto-installs from GitHub releases on first use with SHA-256 checksum verification (and cosign provenance verification if cosign is available).
+
+```yaml
+# In ~/.hermes/config.yaml
+security:
+  tirith_enabled: true       # Enable/disable tirith scanning (default: true)
+  tirith_path: "tirith"      # Path to tirith binary (default: PATH lookup)
+  tirith_timeout: 5          # Subprocess timeout in seconds
+  tirith_fail_open: true     # Allow execution when tirith is unavailable (default: true)
+```
+
+When `tirith_fail_open` is `true` (default), commands proceed if tirith is not installed or times out. Set to `false` in high-security environments to block commands when tirith is unavailable.
+
+Tirith's verdict integrates with the approval flow: safe commands pass through, suspicious commands trigger user approval, and dangerous commands are blocked.
+
 ### Context File Injection Protection
 
 Context files (AGENTS.md, .cursorrules, SOUL.md) are scanned for prompt injection before being included in the system prompt. The scanner checks for: