docs: add agent integration guide; fix L4 deep-link param in README

Add AGENT_GUIDE.md describing how a practice-managing agent drives the
deployed
tool (guitar.basdado.com) via deep links, with the full drill/param
catalog.
Correct the README param table: L4 uses ?note, not ?root.

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

README.md

@@ -105,9 +105,10 @@ Optional query parameters override the stored/default settings:
 | `bpm`      | click-track drills    | `?bpm=66`      |
 | `variant`  | drills with variants  | `?variant=2-3-2` |
 | `key`      | T2, T4                | `?key=D`       |
-| `root`     | T3, T5, T6, L4        | `?root=A`      |
+| `root`     | T3, T5, T6            | `?root=A`      |
 | `interval` | T5                    | `?interval=P5` |
 | `mode`     | T6                    | `?mode=dorian` |
+| `note`     | L4                    | `?note=C`      |
 | `pair`     | R4                    | `?pair=g-c`    |
 
 Examples: