Document the PCSX2 test setup

PCSX2 was set up and working but existed only in this machine's filesystem
and in people's heads, so every comparison looked like a one-off observation
rather than something re-runnable. The setup section in
docs/plan/pcsx2-verification.md now records where the emulator, the BIOS, the
disc, the savestates and the PINE socket are, how a state is captured, and
which command the disc integration tests need -- and AGENTS.md and README.md
point at it from their orientation sections. One operational detail worth
spelling out: the emulator's directory is lowercase (pcsx2), which is not
what people type.

AGENTS.md

@@ -132,6 +132,7 @@ Read these before working in an unfamiliar area:
 | `docs/plan/` | One file per subsystem, written **as work proceeds** |
 | `docs/formats/` | Format specs, written **as formats are reversed** |
 | `PROVENANCE.md` | Upstream projects we build on, and their licences |
+| `docs/plan/pcsx2-verification.md` | PCSX2 is set up on this machine, and how to run the comparison loop |
 | `vendor/reRAC/` | Pinned reference checkout (not tracked) |
 
 ## Crates