Document mining additional chats in the retrofit flow

The playbook assumed one canonical chat per project. Add a merge prompt to
Step 1 for reconciling further conversations into the existing AGENTS.md/
ROADMAP.md without overwriting, and note in the retrofit guide that the
"files exist" skip can't mine other chats — that stays manual.

guides/retrofit.md

@@ -33,7 +33,12 @@ guess on anything that changes what lands on disk.
 
 Check the repo root for `AGENTS.md` with a `CLAUDE.md` symlink pointing at it.
 
-**If both already exist**, Step 1 is done — go to Phase 3.
+**If both already exist**, the first chat has been mined — go to Phase 3. One thing this
+skip can't know is whether *other* conversations are still worth mining: you can't resume
+past chats any more than you could the first one, so if the user has additional chats to
+mine, that stays manual. Point them at the "merge from another chat" prompt in
+`retrofit-playbook.md` Step 1 (it reconciles into the existing files rather than
+overwriting them), then continue at Phase 3.
 
 **If they're missing**, this is the manual seam. Do not proceed and do not invent the
 file. Instead, help the user mine the right chat, then stop:

retrofit-playbook.md

@@ -88,6 +88,12 @@ Then `/exit`. The old chat's job is done forever.
 
 (Caveat: repeated `/compact` has summarized away some early detail, so treat this as a strong draft — the next step checks it against reality.)
 
+**More than one chat worth mining?** The prompts above *create* AGENTS.md and ROADMAP.md, so run A/B/C only on the first chat. For each additional conversation, `claude --resume` it and paste this *merge* prompt instead — it reconciles into the files rather than overwriting them:
+
+> **Merge from another chat:** First read the current AGENTS.md and ROADMAP.md from disk (and any docs/guides/* they reference) so you know what's already captured. Now go through everything we did in *this* conversation and find what those files don't already have: durable facts (stack, commands, conventions, gotchas, decisions), longer-term backlog, and anything that changes what's currently true. Then reconcile — don't overwrite: add genuinely new durable knowledge to AGENTS.md, add longer-term items to ROADMAP.md (create it if it doesn't exist), and update the "## Current state" section only if this chat reflects more recent state than what's there. Where this chat contradicts what's on disk, don't silently pick one — show me both and ask. Then sweep anything you wrote for secrets and replace them with env-var names. Tell me exactly what you added or changed in each file.
+
+Mine every chat you care about now, while they're the perishable asset; then move to Step 2 and verify the merged result once. (A merged-in chat can be just as compacted as the first — Step 2 is still what checks it against reality.)
+
 ### Step 2 — Fresh session: cross-check and verify by running
 
 ```