[agent-guidance] Fix relative links to STYLE.md and AGENTS.md in .gemini/styleguide.md (#10040)

The style guide lives in .gemini/, so links to STYLE.md and AGENTS.md
resolved to .gemini/STYLE.md and .gemini/AGENTS.md, which don't exist.
Point them at the repo-root files instead.

.gemini/styleguide.md

diff --git a/.gemini/styleguide.md b/.gemini/styleguide.md
index 2b088d0..6331445 100644
--- a/.gemini/styleguide.md
+++ b/.gemini/styleguide.md
@@ -35,13 +35,13 @@
 
 Please refer to the following files for specific rules:
 
-* **General Style Context**: See [STYLE.md](STYLE.md) for rules on:
+* **General Style Context**: See [STYLE.md](../STYLE.md) for rules on:
   * Getter and setter order.
   * Naming for typedefs and function variables.
   * Overriding equality.
   * Windows/POSIX path handling conventions.
   * Default text styles.
-* **Repository Constraints**: See [AGENTS.md](AGENTS.md) for rules on:
+* **Repository Constraints**: See [AGENTS.md](../AGENTS.md) for rules on:
   * Constraints on `packages/devtools_shared` (no Flutter dependency).
   * Import restrictions (no importing `devtools_app.dart` from `src/`).
   * Published packages management (CHANGELOGs, versions, breaking changes).