Fix canvas repo detection and shrink oversized preview images (#2227)

Canvas extensions (accessibility-kanban, where-was-i) resolved git state
against the extension's install directory or the session-state folder
instead of the repo the session was opened in. This broke repo/branch
detection for user-scoped installs and worktrees ("unknown/unknown",
"detached HEAD"). Both now read the active session's working directory
from the canvas request context on open and on each action, so they
follow whichever project/worktree opened them.

where-was-i also re-gathers context on open when the saved branch is
blank, so it never opens stuck on stale data.

Also compress oversized extension preview images (pngquant, with a mild
resize for the photographic arcade-canvas art) so every preview.png is
comfortably under the installer's inline-byte cap that was blocking
release-notes-showcase from installing.

Update release-notes-showcase author to Kayla Cinnamon.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

extensions/release-notes-showcase/.github/plugin/plugin.json

@@ -3,8 +3,8 @@
   "description": "Compose and refine launch-ready release notes with contributor callouts and export-friendly output.",
   "version": "1.0.0",
   "author": {
-    "name": "James Montemagno",
-    "url": "https://github.com/jamesmontemagno"
+    "name": "Kayla Cinnamon",
+    "url": "https://github.com/cinnamon-msft"
   },
   "keywords": [
     "changelog",