refactor: rename forgecode agent key to forge

Aligns with AGENTS.md design principle: "Use the actual CLI tool
name as the key, not a shortened version" (AGENTS.md:61-83).

The actual CLI executable is 'forge', so the AGENT_CONFIG key should
be 'forge' (not 'forgecode'). This follows the same pattern as other
agents like cursor-agent and kiro-cli.

Changes:
- Renamed AGENT_CONFIG key: "forgecode" → "forge"
- Removed cli_binary field (no longer needed)
- Simplified check_tool() - removed cli_binary lookup logic
- Simplified init() and check() - removed display_key mapping
- Updated all tests: test_forge_name_field_in_frontmatter
- Updated documentation: README.md

Code simplification:
- Removed 6 lines of workaround code
- Removed 1 function parameter (display_key)
- Eliminated all special-case logic for forge

Note: No backward compatibility needed - forge is a new agent
being introduced in this PR.

src/specify_cli/agents.py

@@ -163,7 +163,7 @@ class CommandRegistrar:
             "args": "$ARGUMENTS",
             "extension": ".md"
         },
-        "forgecode": {
+        "forge": {
             "dir": ".forge/commands",
             "format": "markdown",
             "args": "{{parameters}}",