feat: add read_note start_index pagination

[maehwasoo]

What

• Add start_index + next_start_index pagination to read_note.
• Define EOF behavior and truncated/next_start_index contract for repeated reads.
• Split the monolithic MCP HTTP read-tools test into per-tool test files.
• Update docs for the new pagination API and new test file layout.

Why

• Support reading large notes incrementally without raising max_chars.
• Keep the legacy plain-text content response while adding structured pagination metadata.
• Improve test maintainability by isolating each tool’s behavior.

Fixes N/A

How

• Update packages/mcp/src/tools/getNote.ts to read full text, slice by start_index/max_chars, and compute next_start_index.
• Add/adjust tests under packages/mcp/test/httpTools.*.test.ts (pagination concat + EOF boundary + tool-specific cases).
• Update README.md and docs/ops/testing.md to match the new contract and file layout.
• Validate with pnpm build and pnpm check.