refactor(app): re-export LedMode so apps never import task internals

[ArthurGibert]

Summary

• Apps must only reach hardware-task internals through the App<N> facade in app.rs, never by importing crate::tasks::* directly — this closes the one confirmed gap in that boundary.
• app.rs re-exported leds::Led but not leds::LedMode, even though Leds::set_mode() requires it — three apps (tb3po, lfo, lfo_plus) had worked around this with a direct use crate::tasks::leds::LedMode import to use LED effects.
• Add LedMode to app.rs's existing re-export block, and update those three apps to pull it (and, for lfo/lfo_plus, AppParams/ParamStore) through crate::app:: only.
• Note in AGENTS.md that any new App<N> method taking/returning a crate::tasks::* type must re-export it in the same change, so this doesn't recur.

No behavior change — this only moves import paths and widens a re-export.

Test plan

• On hardware: TB-3PO reseed button flash (channel 0 button) still shows the white flash-then-static LED effect
• On hardware: LFO / LFO+ still display correctly, no LED regressions
• cargo fmt --all -- --check, cargo clippy --bin faderpunk --target thumbv8m.main-none-eabihf -- -D warnings, cargo clippy -p libfp -- -D warnings, cargo test --lib -p libfp all pass (already verified locally)