feat: group session commands under session subcommand

[broody]

Summary

• Combine generate + register into single session auth command (generates keypair then registers in one flow)
• Move status and clear under session subcommand
• Add session list and session revoke stubs
• Remove unused store command
• Warn and prompt user (y/N) when an active session exists; --overwrite skips the prompt
• Flatten status policies output to address:entrypoint array
• Clean up verbose authorize output to show only policies summary, authorization URL, and waiting message
• Reorder top-level commands: session, execute, call, transaction, lookup

Test plan

• cargo build compiles successfully
• cargo test — all 34 tests pass
• controller session auth --file examples/policies.json works end-to-end
• controller session status shows session info with flat policies array
• controller session clear clears session data
• controller session list and controller session revoke print stub messages
• controller --help shows correct command ordering
• controller session --help shows auth, status, list, revoke, clear

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