agency: trailing ✓ suffix on picked button (visibility fix)

[MagMueller]

Quick visibility fix on top of #103.

Symptom

Magnus tapped Yes on a card and asked "is it possible to highlight the button after I press it" — implying he didn't see any change. Bot logs confirmed `editMessageReplyMarkup → 200 OK` fired, so the mark was being applied — just invisible.

Root cause

The mark was a leading `✓ ` prefix. When the button label already starts with a colorful icon (e.g. `✅ Yes`), the prefix collides with it: `✓ ✅ Yes` reads as two checkmarks fighting for the leading slot. On phone-sized button text it's impossible to tell which one is picked.

Fix

Move the mark to a trailing ` ✓` suffix — the conventional "is selected" position. Sits cleanly at the end of any label regardless of leading icon:

Default	Picked
✅ Yes	✅ Yes ✓
⏭ Skip	⏭ Skip ✓
✏️ Edit	✏️ Edit ✓

Also strips legacy `✓ ` prefix on the next tap so cards posted before this change clean up automatically.

Test plan

• Tap a default Yes/Skip/Edit card → trailing ` ✓` shows clearly on the picked button
• Re-tap a different button → previous ` ✓` cleared, new ` ✓` on the latest
• Tap a custom multi-button card → ` ✓` accumulates across taps (additive)

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Summary by cubic

Moved the picked-button mark from a leading ✓ to a trailing ✓ so selections are clearly visible even when labels start with emoji (e.g., ✅ Yes). Also cleans up old prefix marks on the next tap.

• Bug Fixes
  • Keeps tap behavior the same; works across all buttons.
  • Honors reset_others: exclusive for default Yes/Skip/Edit; additive for custom buttons.
  • Strips legacy ✓ prefix and replaces it with ✓ on tap.

^{Written for commit 165b59e. Summary will update on new commits.}