A claimed PC forgets the old login and the old cameras

Seen on the first claimed demo install: 'session expired' on every
screen, signed in as a user from the previous demo's head office, and
'Watching 3 cameras' for a shop with one - the PC had offered its two
leftover cameras up to head office, without their passwords, so the
same lens was listed twice and one copy could never be pushed anywhere.

Claiming now clears any stored session (a new head office is a new
world), a session whose refresh fails is forgotten on disk as well as
in memory so the app returns to Login by itself, and the demo setup
removes cameras left from an earlier install before it joins the shop,
because head office is the source of truth from then on.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KGcjxF1cNLcuwc3DAPcnfj

agent/pkg/paths/paths.go

@@ -70,6 +70,12 @@ func APICredentials() string {
 	return filepath.Join(StateRoot(), "data", "api_credentials.txt")
 }
 
+// CamerasFile is the engine's own camera store. The agent never edits it -
+// cameras go through the engine's API so passwords are sealed - but setup
+// removes it when a PC joins a shop, because head office is the source of
+// truth from then on.
+func CamerasFile() string { return filepath.Join(StateRoot(), "data", "cameras.json") }
+
 // EnsureState creates the writable tree. Called before anything opens a file
 // under it, so a first run on a fresh machine does not fail on a missing dir.
 func EnsureState() error {