feat: add Forge support to shared update-agent-context scripts

- Add forge case to bash and PowerShell update-agent-context scripts
- Add FORGE_FILE variable mapping to AGENTS.md (like opencode/codex/pi)
- Add forge to all usage/help text and ValidateSet parameters
- Include forge in update_all_existing_agents functions

Wrapper script improvements:
- Simplify Forge wrapper scripts to unconditionally delegate to shared script
- Remove complex fallback logic that created stub AGENTS.md files
- Add clear error messages if shared script is missing/not executable
- Align with pattern used by other integrations (opencode, bob, etc.)

Benefits:
- Plan command's {AGENT_SCRIPT} now works for Forge users
- No more incomplete/stub context files masking missing support
- Cleaner, more maintainable code (-39 lines in wrappers)
- Consistent architecture across all integrations

Update AGENTS.md to document that Forge integration ensures shared scripts
include forge support for context updates.

Addresses reviewer feedback about Forge support being incomplete for
workflow steps that run {AGENT_SCRIPT}.

scripts/bash/update-agent-context.sh

@@ -30,12 +30,12 @@
 #
 # 5. Multi-Agent Support
 #    - Handles agent-specific file paths and naming conventions
-#    - Supports: Claude, Gemini, Copilot, Cursor, Qwen, opencode, Codex, Windsurf, Junie, Kilo Code, Auggie CLI, Roo Code, CodeBuddy CLI, Qoder CLI, Amp, SHAI, Tabnine CLI, Kiro CLI, Mistral Vibe, Kimi Code, Pi Coding Agent, iFlow CLI, Antigravity or Generic
+#    - Supports: Claude, Gemini, Copilot, Cursor, Qwen, opencode, Codex, Windsurf, Junie, Kilo Code, Auggie CLI, Roo Code, CodeBuddy CLI, Qoder CLI, Amp, SHAI, Tabnine CLI, Kiro CLI, Mistral Vibe, Kimi Code, Pi Coding Agent, iFlow CLI, Forge, Antigravity or Generic
 #    - Can update single agents or all existing agent files
 #    - Creates default Claude file if no agent files exist
 #
 # Usage: ./update-agent-context.sh [agent_type]
-# Agent types: claude|gemini|copilot|cursor-agent|qwen|opencode|codex|windsurf|junie|kilocode|auggie|roo|codebuddy|amp|shai|tabnine|kiro-cli|agy|bob|vibe|qodercli|kimi|trae|pi|iflow|generic
+# Agent types: claude|gemini|copilot|cursor-agent|qwen|opencode|codex|windsurf|junie|kilocode|auggie|roo|codebuddy|amp|shai|tabnine|kiro-cli|agy|bob|vibe|qodercli|kimi|trae|pi|iflow|forge|generic
 # Leave empty to update all existing agent files
 
 set -e
@@ -690,12 +690,15 @@ update_specific_agent() {
         iflow)
             update_agent_file "$IFLOW_FILE" "iFlow CLI" || return 1
             ;;
+        forge)
+            update_agent_file "$AGENTS_FILE" "Forge" || return 1
+            ;;
         generic)
             log_info "Generic agent: no predefined context file. Use the agent-specific update script for your agent."
             ;;
         *)
             log_error "Unknown agent type '$agent_type'"
-            log_error "Expected: claude|gemini|copilot|cursor-agent|qwen|opencode|codex|windsurf|junie|kilocode|auggie|roo|codebuddy|amp|shai|tabnine|kiro-cli|agy|bob|vibe|qodercli|kimi|trae|pi|iflow|generic"
+            log_error "Expected: claude|gemini|copilot|cursor-agent|qwen|opencode|codex|windsurf|junie|kilocode|auggie|roo|codebuddy|amp|shai|tabnine|kiro-cli|agy|bob|vibe|qodercli|kimi|trae|pi|iflow|forge|generic"
             exit 1
             ;;
     esac
@@ -757,6 +760,7 @@ update_all_existing_agents() {
     _update_if_new "$KIMI_FILE" "Kimi Code"                || _all_ok=false
     _update_if_new "$TRAE_FILE" "Trae"                     || _all_ok=false
     _update_if_new "$IFLOW_FILE" "iFlow CLI"               || _all_ok=false
+    _update_if_new "$FORGE_FILE" "Forge"                   || _all_ok=false
 
     # If no agent files exist, create a default Claude file
     if [[ "$_found_agent" == false ]]; then
@@ -783,7 +787,7 @@ print_summary() {
     fi
     
     echo
-    log_info "Usage: $0 [claude|gemini|copilot|cursor-agent|qwen|opencode|codex|windsurf|junie|kilocode|auggie|roo|codebuddy|amp|shai|tabnine|kiro-cli|agy|bob|vibe|qodercli|kimi|trae|pi|iflow|generic]"
+    log_info "Usage: $0 [claude|gemini|copilot|cursor-agent|qwen|opencode|codex|windsurf|junie|kilocode|auggie|roo|codebuddy|amp|shai|tabnine|kiro-cli|agy|bob|vibe|qodercli|kimi|trae|pi|iflow|forge|generic]"
 }
 
 #==============================================================================