feat(cli): implement backend wiring for agent/plugin/session/tool commands

[Rahul-2k4]

Summary

• replace no-op CLI stubs in crates/mofa-cli agent/plugin/session/tool commands with backend-connected implementations
• add shared backend repository module for state/persistence access and structured errors
• wire tool discovery to foundation built-in tool registry via all_builtin_tools
• add backend unit tests covering agent lifecycle, plugin uninstall persistence, and seeded tool discovery

Verification

• cargo test -p mofa-cli (19 passed)

Notes

• branch rebased onto origin/main to keep PR scope focused on issue feature(cli): Implement missing backend connections for CLI commands #146