fix(lint): code-fence-wrap detector and fixer regex now agree (#1597)

The code-fence-wrap detector in lintContent used the /m multiline flag, so
^/$ matched start/end of any line. The rule fired on any page that simply
contained a ```markdown code block, not only pages wrapped end-to-end.

The matching fixer in fixContent has no /m flag, so it can only strip
whole-file wrappers. Result: detected issues were marked fixable: true,
yet fixContent could never strip them. `gbrain dream` reported
"0 fix(es) applied, N remaining" perpetually for the rule.

Drops the /m flag from the detector so detector and fixer stay in sync.
Whole-file wrapper detection is preserved; inner code blocks no longer
trigger the rule.

Real-world impact: a brain with 5 docs pages containing markdown examples
(skill READMEs, decision registry, journal templates) reports 5 phantom
"fixable: true" issues every dream cycle, never converging. After this
fix, the dream-cycle lint phase reports only real-and-unfixable issues
(missing frontmatter, missing title/type) which is the intended behavior.

Two regression tests added in test/lint.test.ts:
- Page contains a single inner ```markdown block
- Page contains multiple inner ```markdown blocks

Both assert no code-fence-wrap issue is reported. The existing
"detects wrapping code fences" test (true-positive case) continues to
pass; total tests in the file are 18 -> 20.

Co-authored-by: Thomas Chung <thomaschung@macbookair.lan>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: Time Attakc <89218912+time-attack@users.noreply.github.com>

src/commands/lint.ts

@@ -127,7 +127,12 @@ export function lintContent(content: string, filePath: string, opts: LintContent
   }
 
   // Rule: Wrapping code fences (```markdown ... ```)
-  if (content.match(/^```(?:markdown|md)\s*\n/m) && content.match(/\n```\s*$/m)) {
+  // Detector intentionally has NO /m flag so ^/$ match start/end of the whole
+  // file, not inner lines. Keeps detector in sync with fixContent() below,
+  // which also has no /m flag. Without this, lint reports "fixable" false
+  // positives on any page that simply contains a ```markdown code block, but
+  // fixContent can never strip them (its regex only matches whole-file wrappers).
+  if (content.match(/^```(?:markdown|md)\s*\n/) && content.match(/\n```\s*$/)) {
     issues.push({
       file: filePath, line: 1, rule: 'code-fence-wrap',
       message: 'Page wrapped in ```markdown code fences (LLM artifact)',