docs(content): create the content roadmap — Spine, BOM, Backlog

Implements the docs restructure half of the content-track design (the
currency item is code and stays unbuilt). Content stops being unbounded:
the work is now a countable checklist with an end.

content/roadmap.md — new. Three sections, only one of which is ever the
work:
  Spine     Greywater Docks + one dungeon, patron Ghaul the Yoke; the
            cosmology touching ground. Plus what is already canon.
  BOM       ~57 discrete pieces for a complete, playable game, per
            milestone. Numbers, not vibes.
  Backlog   Everything the slice does not need. Not a debt — where ideas
            go to be safe.

Its rule: never author anything that is not in the Bill of Materials.

The BOM carries a Fixed/Variable column — the seven-campaign saga spec's
only ask of this table. ~37 fixed / ~20 variable: a second town costs the
20, not the 57. That is also the exact split v2 generation would need.

content/lore/canon-roadmap.md becomes a pointer (older specs and plans
reference the old path). An items-and-prices table does not belong under
lore/.

docs/roadmap.md — M4..M9 each gain a "Content it consumes →" line into
the BOM.

world-building skill — SKILL.md and evals.json now point at
content/roadmap.md, and the skill is told to respect the BOM rule: ideas
that arrive during authoring go to the Backlog, not into the world.

Build + --check green; generated content/world and content/server are
byte-identical. No Python touched.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.claude/skills/world-building/SKILL.md

@@ -148,8 +148,13 @@ touched those, just confirm they're valid JSON.)
 
 Summarize: new ids added (by layer), any existing id you reused in `related`,
 anything you flagged for approval, and confirmation the build + check + tests are
-green. Update `content/lore/canon-roadmap.md`'s Authored/Pending lists if you
-added a new bible file or closed out a pending thread.
+green. Update [`content/roadmap.md`](../../../content/roadmap.md) if you added a new
+bible file or closed out a thread — its Spine list, or the Bill of Materials line
+you just satisfied.
+
+**Respect its rule: never author anything that is not in the Bill of Materials.**
+An idea that arrives during authoring goes to that doc's Backlog, not into the
+world.
 
 ## Reference files