feat(config): add workflow.subagent_timeout to replace hardcoded 5-minute limit

The map-codebase workflow had a hardcoded 300000ms (5 minute) timeout
for parallel subagent tasks. On large codebases or with slower models
(e.g. GPT via Codex), subagents can need 10-20+ minutes, causing the
parent to kill still-working agents and fall back to sequential mode.

Changes:
- Add workflow.subagent_timeout config key (default: 300000ms)
- Register in VALID_CONFIG_KEYS (config.cjs)
- Add to loadConfig() defaults and return object (core.cjs)
- Emit in map-codebase init context (init.cjs)
- Update map-codebase.md to use config value instead of hardcoded 300000
- Document in planning-config.md reference

Users can now increase the timeout via:
  /gsd:settings workflow.subagent_timeout 900000

Closes #1472

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

get-shit-done/workflows/map-codebase.md

@@ -36,7 +36,7 @@ if [[ "$INIT" == @file:* ]]; then INIT=$(cat "${INIT#@file:}"); fi
 AGENT_SKILLS_MAPPER=$(node "$HOME/.claude/get-shit-done/bin/gsd-tools.cjs" agent-skills gsd-codebase-mapper 2>/dev/null)
 ```
 
-Extract from init JSON: `mapper_model`, `commit_docs`, `codebase_dir`, `existing_maps`, `has_maps`, `codebase_dir_exists`.
+Extract from init JSON: `mapper_model`, `commit_docs`, `codebase_dir`, `existing_maps`, `has_maps`, `codebase_dir_exists`, `subagent_timeout`.
 </step>
 
 <step name="check_existing">
@@ -199,9 +199,11 @@ Wait for all 4 agents to complete using TaskOutput tool.
 TaskOutput tool:
   task_id: "{task_id from Agent result}"
   block: true
-  timeout: 300000
+  timeout: {subagent_timeout from init context, default 300000}
 ```
 
+> The timeout is configurable via `workflow.subagent_timeout` in `.planning/config.json` (milliseconds). Default: 300000 (5 minutes). Increase for large codebases or slower models.
+
 Call TaskOutput for all 4 agents in parallel (single message with 4 TaskOutput calls).
 
 Once all TaskOutput calls return, read each agent's output file to collect confirmations.