Update collaboration doc

Add some diagrams

docs/diagrams/src/login-flow.uml

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+@startuml
+
+participant "Zed App" as app
+participant "Browser" as browser
+participant "Zed HTTP server" as server
+
+rnote over app: Generate encryption keypair.\nListen on an open TCP port.
+app -> browser: Open sign-in page\n(public key, TCP port)
+browser -> server: Sign in\n(app's public key, app's TCP port)
+
+rnote over server: Generate access token.\nEncrypt it with app's public key.
+server -> browser: Redirect to loopback with app's port\n(user id, encrypted access token)
+browser -> app: Follow redirect\n(user id, encrypted access token)
+
+rnote over app: Decrypt access token with private key.\nSave credentials to keychain.
+
+@enduml

docs/diagrams/src/share-worktree.uml

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+@startuml
+
+participant "Zed App" as app
+participant "Zed HTTP server" as server
+participant "Zed RPC server" as rpc_server
+
+app -> server: POST worktrees\n(user id, access token)
+server -> app: OK\n(worktree id, RPC server address)
+
+app -> rpc_server: Connect to the given RPC server address
+app -> rpc_server: **Auth**\n(user id, access token)
+rnote over rpc_server: Load access token hashes for user.\nVerify that one matches the given access token.
+rpc_server -> app: **AuthResponse**\n(credentials valid)
+
+app -> rpc_server: **ShareWorktree**\n(worktree_id, file paths, file digests)
+rnote over rpc_server: Load existing files.\nIdentify which are needed.
+rpc_server -> app: **ShareWorktreeResponse**\n(needed path indices)
+
+@enduml