Tracking: publish the first .mcpb release asset

[millsmillsymills]

Goal

Get an actual shortcut-mcp.mcpb file onto a GitHub Release. The MCPB pipeline
is fully built — this issue tracks the remaining gap between "infrastructure
merged" and "a user can download the bundle."

Spec: https://github.com/modelcontextprotocol/mcpb —
MANIFEST.md,
CLI.md.

Current state

• mcpb/manifest.json (v0.4, uv server type, full user_config for token /
  mode / profile / destructive toggle), mcpb/icon.png,
  scripts/build_mcpb.py (version-synced bundle + registry server.json with
  SHA-256), and .github/workflows/mcpb.yml (build on push/PR; attach
  shortcut-mcp.mcpb + server.json on release: published) are all merged
  (feat: add MCPB bundle build for MCP registry submission #87).
• But the only release, v0.5.0 (2026-05-28), predates feat: add MCPB bundle build for MCP registry submission #87 and has zero
  assets. No release has been published since the attach job existed, so the
  release→attach path has never run for real.
• pyproject.toml is still 0.5.0; dependency bumps and fixes have landed on
  main since (e.g. fix(deps): bump cryptography/python-multipart/starlette/vcrpy (audit CVEs) #119 audit-CVE bumps), so there is releasable content in
  [Unreleased].

Work items

• Cut the next release per RELEASING.md (version bump + CHANGELOG PR,
  signed tag, GitHub Release) — semver call based on the current
  [Unreleased] contents.
• Confirm the mcpb.yml attach job ran on the published release and that
  shortcut-mcp.mcpb and server.json are downloadable from the release
  page.
• Verify the attached bundle: npx @anthropic-ai/mcpb validate on its
  manifest, then install the downloaded .mcpb into Claude Desktop — config
  UI shows the four fields, tools list under the core profile, write and
  destructive tools stay hidden under the default readonly mode.
• README: add an MCPB install section (download the .mcpb from the latest
  release, open with Claude Desktop) — the README currently only documents
  uv tool install from git.
• README ## Status: reflect that the MCPB packaging half of the S3→S4
  gate is done; registry submission (Submit shortcut-mcp to the MCP registry #74) remains.

Acceptance criteria

• The latest GitHub Release has shortcut-mcp.mcpb and server.json attached,
  built by CI (not uploaded by hand).
• A fresh Claude Desktop install from the downloaded file reaches a working
  readonly server with no terminal steps.

Out of scope / follow-ups

• MCP registry submission — tracked in Submit shortcut-mcp to the MCP registry #74 (the server.json this pipeline
  produces feeds directly into it).
• Bundle signing (mcpb sign) — optional per spec.