feat(coding-agent): add /resume slash command for switching sessions

[praneshnikhar]

Closes #679

Adds the /resume slash command to interactive mode, allowing users to switch to a saved session by path or session ID without restarting the process.

• Added /resume to CANONICAL_BUILTIN_SLASH_COMMANDS with argument hint <path|id>
• Added dispatch handler in interactive mode that calls the existing handleResumeSession method
• Added test

Note

Add /resume slash command to switch sessions in interactive mode

Registers /resume <path|id> as a built-in slash command in slash-commands.ts and adds handling in interactive-mode.ts that calls handleResumeSession with the provided argument. Invoking /resume without arguments displays a usage error instead of proceeding.

^{Macroscope summarized 00074b1.}

[ruttybob]

I think it's unnecessary because there is an agent view.