Adding support for Copilot Marketplace plugin (#47)

* Add GitHub Copilot plugin and marketplace manifests for Ponytail

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

* Add Copilot hook adapters and plugin data runtime precedence

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

* Document Copilot plugin install flow and instruction fallback mode

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

* Fix Copilot hooks for native output context and state-only mode tracking

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

* docs: add Copilot CLI namespaced command examples

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

* Collapse Copilot hooks into shared activate/mode-tracker

The Copilot hook files duplicated ponytail-activate.js and
ponytail-mode-tracker.js, differing only in output shape. Move that
difference into writeHookOutput (isCopilot branch) and point
copilot-hooks.json at the shared hooks. Deletes both forks (-73 lines).

Refs #1

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* Align Copilot manifest version to 4.4.0 with cross-manifest parity test

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

* Make Copilot and Codex host detection exclusive in runtime output routing

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

* Add Copilot debt command validation with a pull request acceptance checklist

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

* Removed PR template

* Drop tautological copilot command-form test

The namespaced-form assertion built '/ponytail:ponytail-debt' from two
constants and compared it to itself — it tests string concatenation, not
wiring. The file-exists check above already catches a renamed manifest.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

hooks/copilot-hooks.json

@@ -0,0 +1,21 @@
+{
+  "version": 1,
+  "hooks": {
+    "sessionStart": [
+      {
+        "type": "command",
+        "bash": "node \"${PLUGIN_ROOT}/hooks/ponytail-activate.js\"",
+        "powershell": "node \"${PLUGIN_ROOT}\\hooks\\ponytail-activate.js\"",
+        "timeoutSec": 5
+      }
+    ],
+    "userPromptSubmitted": [
+      {
+        "type": "command",
+        "bash": "node \"${PLUGIN_ROOT}/hooks/ponytail-mode-tracker.js\"",
+        "powershell": "node \"${PLUGIN_ROOT}\\hooks\\ponytail-mode-tracker.js\"",
+        "timeoutSec": 5
+      }
+    ]
+  }
+}