fix: ensure frontmatter can process same content as other agents (#8719)

packages/opencode/test/config/fixtures/empty-frontmatter.md

@@ -0,0 +1,4 @@
+---
+---
+
+Content

packages/opencode/test/config/fixtures/frontmatter.md

@@ -0,0 +1,28 @@
+---
+description: "This is a description wrapped in quotes"
+# field: this is a commented out field that should be ignored
+occupation: This man has the following occupation: Software Engineer
+title: 'Hello World'
+name: John "Doe"
+
+family: He has no 'family'
+summary: >
+  This is a summary
+url: https://example.com:8080/path?query=value
+time: The time is 12:30:00 PM
+nested: First: Second: Third: Fourth
+quoted_colon: "Already quoted: no change needed"
+single_quoted_colon: 'Single quoted: also fine'
+mixed: He said "hello: world" and then left
+empty:
+dollar: Use $' and $& for special patterns
+---
+
+Content that should not be parsed:
+
+fake_field: this is not yaml
+another: neither is this
+time: 10:30:00 AM
+url: https://should-not-be-parsed.com:3000
+
+The above lines look like YAML but are just content.

packages/opencode/test/config/fixtures/no-frontmatter.md

@@ -0,0 +1 @@
+Content