chore(security): remove dead reasoning-offload modules flagged in #1114 (#1132)

The managed-reasoning removal (e5897d03) stripped the CLI/MCP wiring but,
despite its stated intent, left the offload modules and their test suite
behind. The dead code still shipped compiled inside the platform bundles,
and its Windows browser-opener was flagged by a security report (#1114)
for routing the login URL through `cmd /c start`, where cmd re-parses
shell metacharacters. Unreachable since 2026-06-20 and never wired in any
tagged release — but delete it for real: src/reasoning/ (config,
credentials, login, reasoner), __tests__/offload.test.ts, the now-inert
CODEGRAPH_OFFLOAD_DISABLE guard in dynamic-boundaries.test.ts, and the
stale reasoner reference in the FILE_SECTION_PREFIX comment.

Closes #1114

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

src/mcp/tools.ts

@@ -334,8 +334,7 @@ function numberSourceLines(slice: string, firstLineNumber: number): string {
  * extension) stop blowing every header up to H1–H4. The path is bold + a code
  * span so it still reads as a header, and the leading ``**` `` stays a UNIQUE,
  * greppable marker — no other explore line begins with it — that the explore
- * truncation boundary (`handleExplore`) and the offload chunker
- * (`reasoning/reasoner.ts`) both key off to cut on whole file sections.
+ * truncation boundary (`handleExplore`) keys off to cut on whole file sections.
  */
 const FILE_SECTION_PREFIX = '**`';
 // Placeholder for codegraph_explore's "Found N symbols across M files." line.