remove: prompt for purge interactively + offer to clean up the appended .gitignore line

[isingh]

Problem

After install → remove in a project, the experience leaves cruft and the purge option is hard to discover.

• Purge is flag-only. truecast remove <name> confirms the detach, but there's no interactive prompt for purge — you have to already know --purge.
• The appended .gitignore line is never cleaned up. On install we append .truecast/agents/*/core to the project .gitignore; remove never offers to revert it, so git status shows a stray modified .gitignore even after you've removed everything.

Context: surfaced as "not the best experience" — after install then remove in a project, git status shows a modified .gitignore plus a leftover .truecast/.

Desired interactive flow

truecast remove <name> (no --yes):

1. Remove <name>? [y/N]
2. Also delete instance/ (purge)? [y/N]
   • purge = yes → remove the core symlink (detach) and delete instance/, then ask Remove the ".truecast/agents/*/core" line from .gitignore? [y/N] and remove it if confirmed.
   • purge = no → detach only (remove the core symlink + lock entry, keep instance/); do not touch .gitignore.

--yes skips the prompts; --purge preselects purge.

Notes / open questions

• The .gitignore line is a shared glob covering every .truecast/agents/*/core. Only offer to remove it when no other persona still needs it (i.e. this is the last attached persona) — otherwise removing it breaks siblings.
• Keep the programmatic Confirm/consent gate as the single owner. This is a CLI-prompt-shape change (two questions) feeding the existing remove-project consent, plus a new (gated) gitignore-cleanup step.
• Decide whether --purge should still require --yes non-interactively, or whether the interactive purge prompt supersedes that.

Acceptance

• Interactive remove asks remove → purge (two steps).
• Purge offers the .gitignore-line cleanup; declining leaves it.
• Non-purge detach leaves .gitignore untouched.
• --yes / --purge flags preserve current non-interactive behavior.
• Covered by tests (extend the consent conformance + a CLI prompt test).