fix: resolve duplicate skill name causing marketplace validation failure (#204) (#214)

Two SKILL.md files declared `name: last30days` with `user-invocable: true`,
which caused strict marketplace validators to reject the plugin with "Some
plugins in this marketplace have validation errors":

- ./SKILL.md (canonical, also reachable via skills/last30days-nux/ symlink)
- ./skills/last30days/SKILL.md (v3 architecture spec, real file)

In v2.9.6, skills/last30days/SKILL.md was a symlink to ../../SKILL.md so
only one skill existed. Commit 0a9ff16 (v3.0.0) added a new real file at
skills/last30days-v3/SKILL.md, and commit 9be0780 then renamed that
directory to skills/last30days/, replacing the original symlink with a
different real file. The collision has been live since v3.0.0 shipped.

This change:
- Renames skills/last30days/SKILL.md to name: last30days-v3-spec and sets
  user-invocable: false. The file stays in place as internal architecture
  documentation, but it no longer competes with the canonical skill.
- Fixes README.md link that pointed to the deleted skills/last30days-v3/
  path (left over from the rename).
- Removes a stale variants/open/SKILL.md reference (variants/open was
  deleted in v3.0.0).

After the change, only one canonical name=last30days user-invocable=true
skill exists (the root SKILL.md, also reachable via the
skills/last30days-nux/ symlink, same inode).

Closes #204.

This contribution was developed with AI assistance (Codex).

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>

README.md

@@ -12,7 +12,7 @@
 
 **An AI agent-led search engine scored by upvotes, likes, and real money - not editors.**
 
-This README tracks the current v3 pipeline. The runtime skill spec lives in [skills/last30days-v3/SKILL.md](skills/last30days-v3/SKILL.md), which is the source of truth for the latest command and setup behavior.
+This README tracks the current v3 pipeline. The runtime skill spec lives in [skills/last30days/SKILL.md](skills/last30days/SKILL.md), which is the source of truth for the latest command and setup behavior.
 
 Claude Code:
 ```