[FEATURE]: /loop — dynamically repeat a command N times

Requested feature

Add a /loop <command> slash command (inspired by Claude Code's /loop) that repeats a command dynamically — either a fixed count or until a condition is met.

Why

openCode's session.command execution path (packages/opencode/src/command/index.ts) is single-shot: one slash command maps to one template invocation. There is no built-in looping/iteration primitive for rerunning a task until success, until a test passes, or a fixed N times.

Proposed behavior

• /loop 5 <cmd> runs a command 5 times.
• /loop until-pass <cmd> retries a command until it exits successfully.
• Loop state (count, per-iteration result/timestamps) surfaced in the TUI; interruption via the existingsession.interrupt path.

Benefits

• Native retry/iterate workflows for flaky tests, convergence loops, or repeated formatting/lint passes without writing a script.