fix: store recap state in XDG state dir for cross-runtime installs

[jms830]

Problem

The global model override (state/config.json), the blacklist, and the debug log were resolved to a hardcoded ~/.pi/agent/extensions/pi-recap/ path. Two issues:

1. Host-specific. Under oh-my-pi the package installs at ~/.omp/plugins/node_modules/pi-recap, so writing state into ~/.pi/... is surprising, and earlier attempts to keep state inside the package dir get wiped on every bun install/reinstall (node_modules).
2. Not cross-runtime: the same machine running both pi and oh-my-pi had no single, stable home for the override.

Change

Resolve state under the XDG state dir, host-neutrally, in all three files (state/config.ts, state/blacklist.ts, util/log.ts):

PI_RECAP_HOME            (explicit override / e2e tests)
  -> <cwd>/state         (when cwd is the pi-recap project, dev)
  -> $XDG_STATE_HOME/pi-recap   (default ~/.local/state/pi-recap)
  -> legacy ~/.pi/agent/extensions/pi-recap   (read fallback if it already exists)

• Host-neutral: identical resolution under pi and oh-my-pi — no ~/.pi-vs-~/.omp branching.
• Reinstall-safe: state lives outside node_modules.
• Back-compatible: existing pi users keep their current state via the legacy fallback (used whenever the XDG file doesn't exist yet but the legacy one does); fresh installs use XDG.

Notes

• .js files regenerated from .ts with the repo transpile (tsc --module esnext --target es2022 --moduleResolution bundler); unchanged siblings emit byte-identical.
• tsc --noEmit adds no new type errors vs main; ubs reports 0 critical / 0 warning on all three files (unchanged from main).
• CLAUDE.md path references updated to match.
• Verified all three resolution branches (default XDG, fresh-install default, legacy fallback) with a fake HOME/XDG_STATE_HOME harness.