docs: add marksman LSP validation and update documentation

Document marksman Markdown LSP build and validation success.

Changes:
- STATUS.md: Added marksman as validated LSP (2025-12-13, 144MB)
- AGENTS.md: Added marksman to Development Tools section
- AGENTS.md: Updated server counts (Development Tools: 3→4)

Validation:
- Container builds successfully from custom Dockerfile
- Version 2025-12-13 verified with --version flag
- Crush wrapper script functional
- All three LSP servers (bash, docker, marksman) now working

STATUS.md

@@ -21,6 +21,7 @@ Last validated: 2026-01-22
 **Recently Built:**
 - bash-language-server: Built (v5.6.0) - configured in crush.json via wrapper script
 - docker-language-server: Built (v0.0.0) - configured in crush.json via wrapper script
+- marksman: Built (2025-12-13) - configured in crush.json via wrapper script
 
 **Builds in Progress:**
 - None
@@ -31,6 +32,7 @@ Last validated: 2026-01-22
 **LSP Servers:**
 - ✓ bash-language-server: Working (v5.6.0) - stdio-based, configured in crush.json
 - ✓ docker-language-server: Working (v0.0.0) - stdio-based, configured in crush.json
+- ✓ marksman: Working (2025-12-13) - stdio-based, configured in crush.json
 
 ## Detailed Status
 
@@ -38,6 +40,7 @@ Last validated: 2026-01-22
 |------------|--------|-------|
 | bash-language-server | Built | Container built using prebuilt npm package (190MB). LSP configured in crush.json via wrapper script. Version 5.6.0. |
 | docker-language-server | Built | Container built from Go source (49.2MB). LSP configured in crush.json via wrapper script. Version 0.0.0. |
+| marksman | Built | Container built from prebuilt binary (144MB). LSP configured in crush.json via wrapper script. Version 2025-12-13. |
 
 ## Known Issues