feat(cli): home screen echoes tb (the canonical launcher) when it's installed

[LukasWodka]

Summary

The home screen's command menu + examples used to echo whatever name you invoked (tracebloc→tracebloc, tb→tb) plus a separate "type tb instead" tip. Since the installer always places the tb launcher, tb is the canonical everyday command we want to teach — so the menu echoing tracebloc just because you typed the long name taught the wrong habit.

Now the home screen echoes tb whenever a tb launcher exists beside the CLI (the normal installed state), and falls back to the invoked name only when it genuinely doesn't (e.g. a bare go build), so it never prints a tb that wouldn't run. With the menu itself in tb, the redundant "type tb instead" tip is dropped.

Behavior

Invoked as	tb launcher present	Examples show
tracebloc	yes (installed)	tb … ← the change
tb	yes	tb …
tracebloc	no (bare build)	tracebloc … (fallback — never a tb that won't run)

Tests

• TestResolveHomeModel_PrefersTB — mutation-proven both directions (prefers tb when the launcher exists; falls back to the invoked name otherwise) + signed-out.
• Updated the locked-demo golden (TestRenderHome_MatchesLockedDemo) and the state tests to drop the tip.
• make ci green.

Note

This changes the home screen locked in #244 (Lukas's call this session — tip → menu-in-tb). Separate from the semseg work (#247); touches only home.go + its tests.

🤖 Generated with Claude Code

[saadqbal]

LGTM 👍 Nicer than the tip — echoing tb directly in the menu teaches the short command without a nag line. Confirmed tbAliasAvailable is a cheap local stat (false on error), so hoisting it above the sign-in gate is safe on the instant logged-out path, and the fallback never prints a tb that wouldn't run.