fix: increase headless new-milestone timeout and limit investigation scope (#1230)

Two issues with headless new-milestone:

1. Default 300s timeout is too short — codebase investigation + artifact
   writing for a new milestone regularly exceeds 5 minutes. Bumped to
   600s (10 min) when the default hasn't been explicitly overridden.

2. The discuss-headless prompt's 'Investigate' step had no guidance on
   how much time to spend scouting. LLMs would exhaustively explore the
   codebase (50+ tool calls) before writing any artifacts, running out
   of time. Added 'brief' qualifier and 5-6 tool call budget with a
   note that the research phase does deeper investigation later.

The commit_docs: false preference is already respected — the prompt
correctly says 'Do not commit' when commit_docs is false, and
ensureGitignore idempotently skips when .gsd/ is already in .gitignore.

Fixes #1227

src/headless.ts

@@ -178,6 +178,11 @@ async function runHeadlessOnce(options: HeadlessOptions, restartCount: number):
   const startTime = Date.now()
   const isNewMilestone = options.command === 'new-milestone'
 
+  // new-milestone involves codebase investigation + artifact writing — needs more time
+  if (isNewMilestone && options.timeout === 300_000) {
+    options.timeout = 600_000 // 10 minutes
+  }
+
   // Supervised mode cannot share stdin with --context -
   if (options.supervised && options.context === '-') {
     process.stderr.write('[headless] Error: --supervised cannot be used with --context - (both require stdin)\n')

src/resources/extensions/gsd/prompts/discuss-headless.md

@@ -16,13 +16,15 @@ Summarize your understanding of the specification concretely:
 - Scope estimate (how many milestones × slices)
 - Any ambiguities or gaps you notice
 
-### Step 2: Investigate
+### Step 2: Investigate (brief)
 
-Scout the codebase to understand what already exists:
+Quickly scout the codebase to understand what already exists — spend no more than 5-6 tool calls here:
 - `ls` the project root and key directories
 - Search for relevant existing code, patterns, dependencies
 - Check library docs if needed (`resolve_library` / `get_library_docs`)
 
+Then move on to writing artifacts. Do not explore exhaustively — the research phase will do deeper investigation later.
+
 ### Step 3: Make Decisions
 
 For any ambiguities or gaps in the specification: