docs(ai): ai.md becomes a router; invariants move to AGENTS.md; budgets gated

ai.md 228 → 64 lines. It now carries only: what the product is, the build
commands, the architecture table, a "where the truth lives" table, and a
pointer to HANDOFF. No narrative, no status, no duplicated invariants.

AGENTS.md (195 lines) is now the single always-read file:
- the 20 invariants that break output, wipe user data, or ship a legal/trust
  breach — one line each, pointing at the doc that explains it (they replace
  110 lines of invariant stubs that duplicated docs/ai/*.md verbatim)
- Response style + No-Fluff Mode moved in from ai.md (rules belong in the rules file)
- onboarding is now: HANDOFF → ai.md router → the ONE docs/ai file → dir index;
  TASKS.md is read when the question is "what's next", not to write code
- memory rides with the change but is no longer a unit of its own; HANDOFF is
  ≤40 lines and overwritten, never appended

Anti-rebloat gate: scripts/verify.sh gains step 1/4, a doc-budget check
(AGENTS ≤210, ai ≤100; HANDOFF/TASKS caps land with the queue trim). A commit
that pads a capped file instead of moving detail fails the gate.

schema.md and docs/index.md corrected for the new roles and reading order.
Docs-only unit: scope-check run, no build.

docs/ai/index.md

@@ -1,9 +1,9 @@
 # docs/ai — index
 
-Subsystems split out of [`ai.md`](../../ai.md) (2026-10-26) so a session pays only for the
-subsystem its task touches. The split is a **verbatim move**: history, traps and measurements are
-unchanged, and `ai.md` keeps a short stub carrying the load-bearing invariants plus a link here, so
-no rule is reachable only from a file you forgot to open.
+Subsystems split out of [`ai.md`](../../ai.md) so a session pays only for the subsystem its task
+touches. `ai.md` is now a **router** (what the product is, the architecture table, where truth
+lives) and `AGENTS.md` carries the invariants that hold across all of them, so no rule is
+reachable only from a file you forgot to open.
 
 Read the file whose row matches the task. There is no required reading order.