chore(coder): untrack live coder-providers.json, ship example

The live config is read AND written by the coder (UI provider toggles PATCH it),
so tracking it churned `git status`. Untrack it (now gitignored under data/*),
add a tracked data/coder-providers.example.json reference, and update the
.gitignore exception + CLAUDE.md/BOOCODER.md docs. Loader already falls back to
{providers:{}} (built-ins only) when the live file is absent. + CHANGELOG v2.5.15.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

data/coder-providers.example.json

@@ -0,0 +1,12 @@
+{
+  "providers": {
+    "goose": { "enabled": false },
+    "amp-acp": {
+      "extends": "acp",
+      "label": "Amp",
+      "description": "ACP wrapper for Amp",
+      "command": ["amp-acp"],
+      "enabled": true
+    }
+  }
+}