v0.3.5

Patch release. Polishes the list-rendering path: empty columns
are now dropped automatically.

`mainwp clients list` was hardcoding a Status column that the
API does not return for clients, so the table rendered with a
Status header over a column of empty cells. The user had to
know the MainWP clients endpoint does not return a status field
in order to interpret the table.

After this change, `_mainwp_render_list` walks every row
once, collects each column's value, and drops any column whose
values are all empty. The table header and the row content
shrink to match.

`mainwp --plain clients list --no-input` now prints:
ID,Name,Email
5 CREA elianaeiwa@gmail.com
4 Dynamic Dutch info@dynamicdutch.com
1 Jazmín Gonz... shop@jazmingonzalez.pe
3 Luis Cavero lcaverop@yahoo.es

`sites list` still prints Status because every site row has
`status: "connected"`. `users list`, `tags list`, and the
other list endpoints print whatever columns actually have data.

Also: refreshed the `skills/mainwp-cli/SKILL.md` bundle to 0.3.5
with the changes accumulated since 0.3.0 (the bundle had not
been updated with the v0.3.1-0.3.4 fix waves). Agents that load
this skill now have an accurate description of:

• `mainwp skill install` (new subcommand for cross-agent
  skill propagation)
• the `--profile` global flag and profile switching
• `MAINWP_URL` / `MAINWP_API_KEY` env-var precedence
• TTY-driven output mode (gum vs plain) and the `--json`
  vs `--plain` flags
• the four response envelope shapes the API uses
• the common failure modes for `init` and list endpoints

61 smoke tests, all passing.