Add page-markdown tools; source Notion-Version per operation

Adds two tools from the public Notion API that are a natural fit for
agentic clients and were missing from the spec:

- retrieve-page-markdown (GET /v1/pages/{page_id}/markdown)
- update-page-markdown  (PATCH /v1/pages/{page_id}/markdown)

These endpoints read/write page content as enhanced Markdown instead of
block JSON, which is far more token-efficient for LLMs.

They require Notion-Version 2026-03-11, but the rest of the API stays on
2025-09-03. To support that without a breaking global version bump, the
Notion-Version header is now sourced per operation from the OpenAPI spec:

- HttpClient applies each operation's header-parameter `default` as a
  request header (resolving $ref'd params), so each endpoint pins the
  version it needs. An explicitly-configured Notion-Version (via
  OPENAPI_MCP_HEADERS) still wins.
- The proxy no longer hardcodes Notion-Version for the NOTION_TOKEN path;
  it comes from the spec.
- The parser no longer exposes header params (e.g. Notion-Version) as
  model-visible tool inputs — they were inert noise on 21/22 tools.

Tests: per-operation header injection + user-override precedence
(http-client.headers.test.ts), and markdown tool generation + no
Notion-Version leakage against the real spec (notion-markdown-tools.test.ts).
Full suite: 86 passing.

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

Author: ksinder

README.md

@@ -69,6 +69,17 @@ If you have hardcoded tool names or prompts that reference the old database tool
 
 ---
 
+## Page content as Markdown
+
+The server exposes two tools for working with page content as enhanced Markdown instead of block JSON, which is significantly more token-efficient for AI agents:
+
+- `retrieve-page-markdown` — Read a page's full content as Markdown (`GET /v1/pages/{page_id}/markdown`). Pass `include_transcript: true` to inline meeting-note transcripts.
+- `update-page-markdown` — Edit a page's content with Markdown (`PATCH /v1/pages/{page_id}/markdown`). Prefer `replace_content` to overwrite the whole page, or `update_content` for targeted find-and-replace edits.
+
+These endpoints require Notion API version `2026-03-11`. The server now sources the `Notion-Version` header **per operation** from the OpenAPI spec, so these tools use `2026-03-11` while the rest of the API continues to use `2025-09-03` — no configuration needed. If you set `Notion-Version` yourself via `OPENAPI_MCP_HEADERS`, your value takes precedence for every tool.
+
+---
+
 ### Installation
 
 #### 1. Setting up integration in Notion