v0.17 polish: shell environment, screensavers, known-tool discovery

[raiseCatError]

Summary

Final v0.17 polish on top of post-#312 dev. The maintainer physically exercised the earlier build (prompt and screensaver restoration, Shell Environment, known-missing commands, tldr, Black Hole); the items marked "tests only" below were verified by automated tests.

• Panel position is its own setting (panelPosition, Bottom by default, Top optional), independent of the composer position, in Settings → General and in Setup's Start step (the draft previews immediately). Malformed or missing values fall back to Bottom. The shared screen plan places every panelRows surface accordingly.
• Setup owns a clean screen: the ordinary transcript/welcome is not drawn behind Setup Cat (presentation only; nothing is cleared).
• Stale copy: the obsolete "Planned for v0.4" block and its Ask coverage entries are gone.
• Circletastic: glyphs now spiral into their circle as a vortex; each ring is already turning when it forms (no snap), then per-circle catch-up, collapse and sequential supernova blasts as before; progressive 40/62/85/97% harvest with restore.
• Black Hole consumes every captured glyph through one global gravity field (closed-form spiral beyond the simulation cap).
• Cursor travel trail (Motion → "Cursor travel trail", On for fresh installs, Off for migrated configs): a soft, bounded trail on multi-cell caret jumps; presentation only, retargets cleanly, gated by Reduced Motion.
• Progress bars use the shared travelling shimmer: a wave inside the factual fill, a travelling segment when indeterminate, theme roles only, static for Reduced Motion, NO_COLOR and finished tasks.
• Gallery: r replays the screensaver preview from a pristine snapshot (the other animated previews are driven by the shared clock and have no replay state; not changed).
• Earlier in this PR: automatic updates, startup kill, known-tool discovery, screensavers, Shell Environment, host and Linux/WSL work.

Verification

npm run verify:fast and npm run verify (1554 tests, 0 failures) pass locally. verify:release runs on the release commit. Physical validation of iTerm2/Kitty/WezTerm/Windows Terminal and Linux/WSL remains tracked in #310 and #311.