feat: support Pi / OMP sessions and add a Pi mascot

[yourconscience]

Problem

Pi support was half-finished: the repo already had pi / omp hook installers and a pi.png icon asset, but Pi sessions still had no mascot, OMP aliases were inconsistent, and OMP sessions could disappear because the process matcher only knew about pi binaries.

What this PR changes

• add a dedicated PiView mascot and wire pi into the notch icon / mascot routing
• normalize omp, oh-my-pi, and oh my pi to the canonical pi source and show the label as Pi
• make the Pi / OMP extensions resend SessionStart if they attach after the runtime session already exists
• extend Pi process matching so live OMP sessions can stay monitored
• document Pi / Oh My Pi in the README
• add focused core tests for Pi / OMP aliasing and event flow

Preview

Verification

• swift test --filter PiAgentEventFlowTests
• launched the app via the debug binary and verified that a live OMP session appears in ~/.codeisland/sessions.json when the Pi / OMP extension is imported and runs through a prompt/tool cycle

Notes

There is no matching issue or contribution template in this repo, so I kept the change narrowly scoped and put the rationale directly in this PR.