feat(attachments): add upload/delete/list_attachments tools

[jacky1967]

Summary

Adds 3 new MCP tools to the Obsidian backend for binary file management (PDFs, images, audio attachments). Designed for consumers that prefer a single MCP-driven write path over a direct filesystem fallback for non-Markdown content.

Motivation

Current MCP tools cover Markdown pages but not binary attachments. Consumers wanting to operate strictly through MCP have to fall back to a separate filesystem layer for things like image uploads or PDF storage, which forces them to maintain two distinct write paths and complicates path-traversal hardening.

What's added

3 tools on *vault.Client (Obsidian-only by design — Logseq has no filesystem concept, so not added to backend.Backend):

• upload_attachment(path, contentBase64) — atomic temp+rename, auto-creates parent dirs, rejects .md (caller should use create_page).
• delete_attachment(path) — rejects .md, errors if missing or if target is a directory.
• list_attachments(folder, recursive?) — non-.md filter, sorted by path, returns size + RFC3339 mtime.

Registered in server.go alongside the existing Obsidian reload tool via type-assertion (skipped automatically in --read-only mode).

Path safety

Reuses the existing safePath() helper so attachments inherit the same boundary checks as pages — no new attack surface.

Tests

ok  	github.com/skridlevsky/graphthulhu/tools	0.407s
ok  	github.com/skridlevsky/graphthulhu/vault	3.323s

• 14 tests at the vault layer (atomicity, parent-dir creation, .md rejection on upload/delete, missing path, directory-as-target, listing, recursion, sort order).
• 10 tests at the tools layer (MCP arg validation, base64 decode errors, surface coverage).
• End-to-end smoke over stdio JSON-RPC: initialize → tools/list shows the 3 new tools (35 total) → upload/list/delete/list-empty chain → .md rejection path. (Smoke run locally during development, not committed.)

No side-effect

• Logseq backend untouched (no Backend interface change).
• Existing tools unchanged.
• --read-only mode still skips all writers, including the 3 new ones.
• No new dependency added to go.mod.

Files

 server.go                 |  15 +++
 tools/attachments.go      |  70 ++++++++++++++
 tools/attachments_test.go | 160 +++++++++++++++++++++++++++++++
 types/tools.go            |  16 ++++
 vault/attachments.go      | 181 +++++++++++++++++++++++++++++++++++
 vault/attachments_test.go | 238 ++++++++++++++++++++++++++++++++++++++++++++++
 6 files changed, 680 insertions(+)

Compatibility note vs PR #11

This branch is independent of #11 (Dockerfile). They touch disjoint paths and can be merged in any order.