docs+tools: working-file policy, lane pruner, and a human MD explainer

Two pieces of work.

1. Keep milestones, not steps. Staged NN_*.py lanes were saving a full
   ~80 MB .blend per attempt, so Lena's lane reached 3.0 GB of which 2.2 GB
   was 30 .blend files -- five snapshots to land one crotch fix, five more
   for the bra. The .py recipes are the real history; the blends are cache.

   - .agents/rules/working-files.md: four-tier policy (KEEP / MASTER /
     SCRATCH / UNKNOWN) + how to work a lane.
   - tools/prune_lane.py: classifies a lane and prunes the scratch tier.
     Dry-run by default. Reads a per-lane .lanekeep manifest, flags
     binaries byte-identical to a registered original (canonical name
     always survives a duplicate pair), and never auto-deletes a .blend
     with no step script beside it -- those cannot be rebuilt.
   - .gitignore: scratch patterns can never be committed.

   Dry run on characters/female/lena_nude reports 2.3 GB reclaimable.
   Not applied -- that lane had a live Blender session at the time.

2. .humans/marvelous-designer.html: how we author garments in Marvelous
   Designer, written for people rather than agents -- the six-step process,
   what has been made, the traps that cost hours, and what is still
   unsolved. Matches the .humans/ HTML convention in ariki-game.

Also committing the docs the AGENTS.md knowledge map and the new page
reference, so they are not dangling: the clothing-lane architecture page,
the marvelous-designer skill, and the two screenshots the page embeds.

characters/ is deliberately untracked and stays that way -- it holds GBs of
blends and GLBs, and .gitattributes does not LFS-track .blend.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.humans/README.md

@@ -0,0 +1,16 @@
+# .humans/ — pages written for people, not agents
+
+Self-contained HTML pages explaining how parts of this repo work, for reading in
+a browser. Same convention as `ariki-game/.humans/`.
+
+The operating detail lives in `.claude/skills/` and `.agents/` and is written for
+whoever is driving the tools. These pages are the **explanation** — what we built,
+why it's shaped that way, and what it cost to learn. Where the two disagree, the
+skill/wiki is the source of truth.
+
+| page | about |
+|---|---|
+| `marvelous-designer.html` | How we author Ariki's garments as real sewn cloth in Marvelous Designer — the process, what's been made, the traps, what's still unsolved. Written for Özlem and Jeremy. |
+
+Images are referenced relatively (`../tools/tailor/screenshots/…`), so open these
+from inside the repo rather than copying the file out on its own.