install: tell the user to commit .truecast/ after attaching to a project

[isingh]

Problem

After truecast install <source> in a project, the success output says nothing about what to do with the files it created. The intended model is:

• commit .truecast/instance/ (mandate + work + research) and .truecast/lock — that's the persona's job + pin, shared with your team;
• the .truecast/agents/*/core symlink is gitignored (machine-specific, regenerated on install).

But the CLI never says this, so a git status showing a modified .gitignore + a new untracked .truecast/ reads as "something went wrong" rather than "commit this." (Surfaced as "not the best experience.")

Desired

On a project attach (not --global, not --dry-run), print a one-line hint to stderr after success, e.g.:

✓ installed product-manager@1.0.0
  → commit .truecast/ to share this persona's setup with your team (the core symlink is gitignored)

Notes

• CLI presentation only — the library still returns data, no console. Render in renderInstall.
• Only when result.plan.projectRoot is set (skip for --global).
• Pairs with remove: prompt for purge interactively + offer to clean up the appended .gitignore line #1 (the remove/gitignore experience).

Acceptance

• A project install prints the commit hint; --global and --dry-run do not.