fix: batch fixes for issues #2906, 2909, 2911, 2913, 2915, 2919, 2921, 2924, 2927, 2928 [skip ci]

[echobt]

Summary

This PR implements fixes for 10 bounty issues:

Issue Fixes

• #2906: Add unified --mode filter flag to cortex agent list command (accepts comma-separated values like primary,subagent, keeps legacy --primary/--subagents for backwards compatibility)
• #2909: Add security validation for MCP server environment variables (warns about dangerous patterns like command injection sequences, PATH modifications, checks for sensitive env vars)
• #2911: Fix debug file path normalization (removes trailing . when running cortex debug file ., added normalize_path() helper function)
• #2913: Change --attach flag behavior from silent fallback to explicit error (validates URL format, provides clear error message instead of falling back to local execution)
• #2915: Add --quiet/-q flag to login command (suppresses success output, only shows errors, useful for CI/CD scripting)
• #2919: Add JSON output format to scrape command (extracts structured metadata including title, description, links, images, and meta tags)
• #2921: Add --dry-run flag to cortex mcp add command (previews configuration without making changes)
• #2924: Add capability filters to models list command (--vision, --tools, --json-mode flags to filter by model capabilities)
• #2927: Enhance MCP debug error messages to show full error chain (includes OS error codes and helpful hints for common errors like "not found" or "permission denied")
• #2928: Add cortex agent test command for configuration validation (validates agent config including temperature ranges, tool references, conflicting settings, outputs errors and warnings)

Files Changed

• cortex-cli/src/agent_cmd.rs - Mode filter, Test command
• cortex-cli/src/mcp_cmd.rs - Env var validation, dry-run, enhanced error messages
• cortex-cli/src/debug_cmd.rs - Path normalization
• cortex-cli/src/run_cmd.rs - Attach URL validation
• cortex-cli/src/login.rs - Quiet flag
• cortex-cli/src/main.rs - Quiet flag propagation
• cortex-cli/src/scrape_cmd.rs - JSON metadata output
• cortex-cli/src/models_cmd.rs - Capability filters

Closes #2906, #2909, #2911, #2913, #2915, #2919, #2921, #2924, #2927, #2928