feat(plugin-sdk): one-command publishing — pack, entry, release

Publishing a plugin meant hand-building the zip, running shasum + stat,
resolving the tag's commit, and hand-writing the whole registry entry.
The SDK does all of it now:

- `trek-plugin pack` builds plugin.zip in the exact layout the installer
  reads (own tiny zip writer, so the SDK stays dependency-free and the
  format can't drift from the reader), enforces the same native-binary and
  size rules, and prints the sha256 + size. docs/ is left out — the store
  fetches the screenshot from the repo, so it doesn't belong in the install
  artifact (Koffi's went from 943 KB to 15 KB).
- `trek-plugin entry` emits the ready-to-PR registry entry from the manifest
  + the packed zip + the git tag: commitSha (deref'd), downloadUrl, sha256,
  size, and minTrekVersion derived from the manifest's trek range. `--merge`
  prepends a new version onto an existing entry for updates.
- `trek-plugin release` chains pack → gh release → entry.

Also: the scaffold now points the README at docs/screenshot.png (the path
the store actually fetches, was screenshot-1.png) with a size hint, and
stops hard-coding an MIT license — a plugin is the author's own code under
their own license. Round-tripped against the real server extractor; 18 tests.

plugin-sdk/src/cli/create.ts

@@ -81,7 +81,7 @@ function readme(name: string): string {
 
 > One sentence: what this plugin does.
 
-![screenshot](./docs/screenshot-1.png)
+![screenshot](./docs/screenshot.png)
 
 ## What it does
 
@@ -89,7 +89,9 @@ Describe the feature this plugin adds to TREK.
 
 ## Screenshots
 
-Show it in context (at least one image).
+Show it in context. Commit a \`docs/screenshot.png\` — it's what the store card
+shows. A 16:9 image (e.g. 1600×900) with your plugin centred and some margin
+looks best (the card crops the edges).
 
 ## Permissions
 
@@ -103,7 +105,8 @@ How to configure it.
 
 ## License
 
-MIT
+Your plugin is your own code — license it however you like; TREK does not impose
+one. Replace this line with your license (for example, MIT).
 `;
 }