opentui: add toast & port CLI flags (-c, -s, -m --agent)

[veracioux]

Implements:

• --continue option
• --session option
• --model option
• --agent option
• A toast component (needed for proper and informative error-handling). See screenshots from opentui: add toast & port flags (-c, -s) #3425
• Validation of model and agent (side effect: fixes a crash when you TAB-cycle through agents and happen to pick one with a nonexistent model)
• Error toast in case the session can't be shared

Closes #3372.

@rekram1-node With this PR, all the options except --prompt are implemented - decided to wait on that one because of the opentui-textarea branch.

Test matrix for the --model and --agent options:

• bun dev
• bun dev --agent existing
• bun dev --agent nonexistent # warning toast
• bun dev --agent existing-with-wrong-model # warning toast
• bun dev --model existing
• bun dev --model existing --agent existing
• bun dev --model existing --agent nonexistent # warning toast
• bun dev --model existing --agent existing-with-wrong-model # warning toast
• bun dev --model nonexistent # warning toast
• bun dev --model nonexistent --agent existing # warning toast
• bun dev --model nonexistent --agent nonexistent # warning toast
• bun dev --model nonexistent --agent existing-with-wrong-model # warning toast