feat: rename marketplace to vllnt (0.1.0-canary.5)

[bntvllnt]

Summary

• Renames the marketplace from vllnt-stack to vllnt for Claude, Codex, and Cursor through one generator constant. Installs now read vstack@vllnt, and skill commands stay /vstack:*.
• README: updates the install and update commands. Adds a note that earlier installs must remove the old vllnt-stack marketplace and add it again. Historical evidence keeps the original name.
• Bumps to 0.1.0-canary.5 and adds a CHANGELOG entry.

Verification

• 38 tests OK, including a new assertion that all three catalogs are named vllnt. The local run needed TMPDIR=/var/tmp because /tmp ran out of inodes on this machine; this is unrelated to the change.
• sync.py --check: 413 files, no drift. git diff --check clean. claude plugin validate passes for the catalog and plugin.

https://claude.ai/code/session_01QibzbuYP9NMPNKJdURfxfE