pi-plan -> pi-goals: rename package, command, and file to goals.md

Distinguishes this from the other pi-plan extensions by foregrounding what's
different (goals tracked to verified completion). Mechanical rename only, no
behavior change:
- package @wassname2/pi-plan -> @wassname2/pi-goals (+ repo url)
- plan.md -> goals.md (the canonical file)
- command /plan -> /goals
- file H1 marker "# Plan:" -> "# Goals:", widget/session labels likewise
- internal state keys pi-plan-* -> pi-goals-*

Internal source filename (plan-file.ts) and identifiers (planDrafting, PlanDoc,
setGoalStatus) keep "plan"; they're not user-visible. External burneikis/pi-plan
references are left intact.

Co-Authored-By: Claudypoo <288921227+claudypoo@users.noreply.github.com>

package.json

@@ -1,13 +1,13 @@
 {
-  "name": "@wassname2/pi-plan",
+  "name": "@wassname2/pi-goals",
   "version": "0.0.1",
-  "description": "One plan.md: set goals via plan mode, work them, sign off only when a read-only check passes. Successor to pi-lgtm.",
+  "description": "One goals.md: set goals in plan mode, work them, sign off only when a read-only check passes. Successor to pi-lgtm.",
   "author": "wassname",
   "license": "MIT",
   "type": "module",
   "repository": {
     "type": "git",
-    "url": "https://github.com/wassname/pi-plan.git"
+    "url": "https://github.com/wassname/pi-goals.git"
   },
   "keywords": [
     "pi-package",