Add maintainer skills for IPC, Pyodide, and redux-observable epochs

[jdpigeon]

What

Adds four agent-facing skills under .claude/skills/ that document the non-obvious, cross-cutting architecture a maintainer needs before touching it. They consolidate and upgrade scattered context (and point to .llms/learnings.md for deep detail rather than duplicating it).

• electron-ipc-architecture — the main/preload/renderer process-separation mental model: the no-Node-in-renderer rule, the three message directions, lazy native-module loading.
• electron-ipc-channel — step-by-step procedure for adding/editing/removing an IPC channel without leaving it half-wired (the four files that must stay in sync, invoke vs send vs webContents.send).
• pyodide-mne — how MNE runs in the Pyodide web worker: the JS↔worker↔Python message protocol, async plot routing, package install via InstallMNE.mjs, and prod-only load gotchas.
• redux-observable-epochs — epic anatomy and the numeric marker-code contract linking live collection to MNE analysis (the source of empty-ERP bugs).

Notes

Docs only — no runtime code touched. Grounded in actual file paths/patterns (pyodideEpics.ts, lslBridge.ts, markerRegistry.ts, native.ts, etc.).

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