fix: clarify pnpm reset must be run from main

[jknight]

Issue reported here: https://discord.com/channels/1316387060357140520/1489805152482492586

The helper pnpm reset fails if users are on live-run-through, which they could be if they're poking around the codebase:

$ pnpm reset

> cadence-course-platform@ reset /Users/jknight/Projects/cohort-003-project
> ai-hero-cli reset --branch=live-run-through --upstream=https://github.com/ai-hero-dev/cohort-003-project.git

From https://github.com/ai-hero-dev/cohort-003-project
 * branch            live-run-through -> FETCH_HEAD
error: cannot delete branch 'live-run-through' used by worktree at '/Users/jknight/Projects/cohort-003-project'
fatal: a branch named 'live-run-through' already exists
Unexpected error: FailedToTrackBranchError: Failed to track branch.
 ELIFECYCLE  Command failed with exit code 1.

This PR adds a little hint that they should switch back to main before running pnpm reset:

$ pnpm reset

> cadence-course-platform@ reset /Users/jknight/Projects/cohort-003-project
> git branch --show-current | grep -qx main && ai-hero-cli reset --branch=live-run-through --upstream=https://github.com/ai-hero-dev/cohort-003-project.git || (echo 'Error: pnpm reset must be run from the main branch. Run: git checkout main' && exit 0)

Error: pnpm reset must be run from the main branch. Run: git checkout main