fix: resolve unbound variable and duplicate file update issues

- Fix undefined FORGE_FILE variable in bash update-agent-context.sh
  - Add missing FORGE_FILE definition pointing to AGENTS.md
  - Update comment to include Forge in list of agents sharing AGENTS.md
  - Prevents crash with 'set -u' when running without explicit agent type

- Add deduplication logic to PowerShell update-agent-context.ps1
  - Implement Update-IfNew helper to track processed files by real path
  - Prevents AGENTS.md from being rewritten multiple times
  - Matches existing deduplication behavior in bash script

- Prevent duplicate YAML keys in Forge frontmatter injection
  - Check for existing 'name:' field before injection in both scripts
  - PowerShell: Parse frontmatter to detect existing name field
  - Bash: Enhanced awk script to check frontmatter state
  - Future-proofs against template changes that add name fields

All scripts now have consistent behavior and proper error handling.

scripts/bash/update-agent-context.sh

@@ -74,7 +74,7 @@ AUGGIE_FILE="$REPO_ROOT/.augment/rules/specify-rules.md"
 ROO_FILE="$REPO_ROOT/.roo/rules/specify-rules.md"
 CODEBUDDY_FILE="$REPO_ROOT/CODEBUDDY.md"
 QODER_FILE="$REPO_ROOT/QODER.md"
-# Amp, Kiro CLI, IBM Bob, and Pi all share AGENTS.md — use AGENTS_FILE to avoid
+# Amp, Kiro CLI, IBM Bob, Pi, and Forge all share AGENTS.md — use AGENTS_FILE to avoid
 # updating the same file multiple times.
 AMP_FILE="$AGENTS_FILE"
 SHAI_FILE="$REPO_ROOT/SHAI.md"
@@ -86,6 +86,7 @@ VIBE_FILE="$REPO_ROOT/.vibe/agents/specify-agents.md"
 KIMI_FILE="$REPO_ROOT/KIMI.md"
 TRAE_FILE="$REPO_ROOT/.trae/rules/AGENTS.md"
 IFLOW_FILE="$REPO_ROOT/IFLOW.md"
+FORGE_FILE="$AGENTS_FILE"
 
 # Template file
 TEMPLATE_FILE="$REPO_ROOT/.specify/templates/agent-file-template.md"