fix(build): ship + CI-build @docmost/prosemirror-markdown; refresh AGENTS.md (#333 conformance)

Two infra blockers from the #326-steps-2-5 conformance check — the converter/canon
are correct, but the new shared package wasn't wired into Docker/CI.

BLOCKER 1 (prod): the Docker installer stage copied mcp/build + editor-ext but NOT
packages/prosemirror-markdown. mcp now depends on it (workspace:*) and EAGER-imports
it at runtime — the in-app ai-chat DocmostClient loads build/index.js ->
lib/markdown-converter.js — so the shipped image would resolve a broken workspace
symlink and every ai-chat tool would die with ERR_MODULE_NOT_FOUND. Now the
installer COPYs packages/prosemirror-markdown/build + package.json before the prod
install. (git-sync has no runtime consumer yet — revisit at step 6 with #119.)

BLOCKER 2 (CI red): test.yml/develop.yml build only @docmost/editor-ext before
`pnpm -r test`. That is plain pnpm, which does NOT honour nx `dependsOn: ^build`,
so the package's (gitignored) build/ never appears and its consumers fail:
mcp `pretest: tsc` -> TS2307 Cannot find module '@docmost/prosemirror-markdown',
git-sync vitest typecheck the same. The green local runs only happened because the
coder+reviewer had a full install+build. Added a `pnpm --filter
@docmost/prosemirror-markdown build` step before `pnpm -r test` (mirrors the
editor-ext step); verified the build is clean (tsc exit 0).

Docs (remark 3): AGENTS.md:203 and :285 still told contributors to keep mcp's own
vendored schema mirror "in sync manually" — that copy was deleted by this PR.
Updated both: the converter + schema mirror now live in the SINGLE package
@docmost/prosemirror-markdown (consumed by mcp + git-sync, do NOT reintroduce a
per-package copy); editor-ext is the upstream schema source; the serializer-contract
test guards the boundary. Added the package to the workspace table.

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

.github/workflows/test.yml

@@ -72,6 +72,14 @@ jobs:
       - name: Build editor-ext
         run: pnpm --filter @docmost/editor-ext build
 
+      # @docmost/prosemirror-markdown is the shared converter (#293/#326); its
+      # build/ is gitignored, and plain `pnpm -r test` does NOT honour nx
+      # `dependsOn: ^build`, so its consumers (mcp `pretest: tsc`, git-sync vitest
+      # typecheck) fail with TS2307 Cannot find module '@docmost/prosemirror-markdown'
+      # unless it is built first. Build it before the recursive test run.
+      - name: Build prosemirror-markdown
+        run: pnpm --filter @docmost/prosemirror-markdown build
+
       - name: Run unit tests
         run: pnpm -r test