Add MCP content ladder

Why

The MCP adapter is token-efficient for list and search responses, but large record bodies can still become a dead end for clients that cannot reliably inspect structuredContent or expand custom transcript markers. PDPP needs a standard, grant-scoped path from compact MCP results to the full authorized record or field without dumping large bodies into every tool response.

What Changes

• Define a content ladder for MCP record results: compact content[] preview, canonical structuredContent, bounded field-window read, and MCP resource links.
• Add the generic read_record_field bounded read tool for record fields so clients without reliable resources/read support can still inspect long bodies.
• Add pdpp://record/{handle} and pdpp://field-window/{handle} resource templates so resource-aware clients can use standard resource_link and resources/read flows.
• Require truncation metadata, continuation cursors, and model-visible next steps whenever a field preview is incomplete.
• Prohibit opaque-only expansion markers as the sole representation of record content.
• Add compatibility tests for content-only, structuredContent-aware, and resource-aware MCP clients.

Capabilities

New Capabilities

None.

Modified Capabilities

• mcp-adapter

Removed Capabilities

None.

Impact

• Affects packages/mcp-server tool schemas, result formatting, resource registration, and tests.
• Requires a hard substrate decision before adapter implementation: if existing resource-server APIs cannot return grant-enforced bounded field windows, add that resource-server path before wiring MCP reads.
• Does not change grant semantics, connector manifests, owner-control routes, or source collection behavior.
• Keeps existing schema, query_records, aggregate, search, and fetch behavior compatible while adding a standard continuation path for long fields.