feat(cli,core): trace the keys that open My Files in pdfs diagnose

When the session is stored and unlocked but the daemon does not answer,
pdfs diagnose asks Proton for a key report and prints a Keys section:
each account key, each address key and the account key its token is
for, and the address key the My Files share is encrypted to. It holds
key IDs only, so it can be posted in an issue.

Refs #40

docs/CHANGELOG.md

@@ -11,6 +11,12 @@ scratch (user data in `staging/` and `recovery/` is never touched by this).
 
 ## [Unreleased]
 
+### Added
+- **`pdfs diagnose` names the key that keeps My Files closed.** When you are signed in and the
+  service does not answer, a new **Keys** section asks Proton which account and address keys
+  open, which account key each address key depends on, and which address key the My Files share
+  is encrypted to. It shows key IDs only, no secrets (#40).
+
 ## [3.5.0] — 2026-10-16
 
 Schema: **44**. Queued changes get an index by kind and name, and each synced file remembers the