docs: index Split Payments and the Glossary in llms.txt

[yingzhehu-TT]

Summary

Add specification/split-payments.md and specification/glossary.md to the
llmstxt sections allowlist so both reach the published llms.txt.

Motivation

plugins.llmstxt.sections is a strict allowlist. Comparing it against nav:

• specification/split-payments.md is the only one of the 45 nav pages
  missing from it. git log -S traces this to feat: Introduce the split payments extension #409, which added the nav entry
  without the llmstxt entry; later feature PRs (feat: Add Payment Authentication Actions #458, feat: shopping permalink capability #523) added both, so this
  is a lone omission rather than a policy.
• specification/glossary.md is in neither, and has been since docs: add centralized glossary and acronym standards #241 — while
  being extended by feat: extension-defined Actions primitive #582, feat: reserve the ucp protocol namespace, add map_order for registry ordering #657 and feat!: define sale-basis steps for quantities #653. It is reachable only through a prose
  link at overview.md.

AGENTS.md §2 asks for exactly this: "Sync any MkDocs navigation additions
(mkdocs.yml) with the llmstxt plugin section to ensure that content is
discoverable and legible by agents."

Confirmed against the built artifact before the change:
dev.ucp.shopping.split_payments appears twice in llms-full.txt and zero
times in llms.txt; same for the Glossary.

Scope

One file, two commits — the split-payments entry is a drift fix under the
AGENTS.md rule, while indexing the Glossary is a new editorial decision, so
they are separable if you want only one.

Two things checked and deliberately left alone:

• specification/embedded-protocol.md appears in both the Checkout and Cart
  sections, and in neither nav entry. Left as-is: it is a shared transport page
  referenced by both capabilities, so the duplication reads as intentional and
  keeps each capability index self-contained. It does cost ~30KB of duplicated
  content in llms-full.txt; happy to dedupe in a follow-up if you prefer.
• The Glossary is not added to nav. Whether an orphan page belongs in site
  navigation is an IA decision rather than an index-drift fix (there is precedent
  for llms.txt-only pages: embedded-protocol.md). Happy to add the nav entry
  too if you want it.

Validation

mkdocs build --strict exit 0 under both DOCS_MODE=spec and DOCS_MODE=root.

llms.txt diff against main is exactly the two new lines (86 → 88); the
llms-full.txt diff is purely additive with no other page affected; and the
DOCS_MODE=root output is byte-identical to main.

ucp-schema lint source/ 100 files passed; validate_examples.py 302 passed,
0 failed. yamllint reports no new diagnostic on the added lines.

Note for release branches: split-payments.md does not exist on any release
branch, and sections entries pointing at a missing page fail --strict, so
these two lines should not be cherry-picked.