fix(memory): honor PI_CODING_AGENT_DIR for user-scope memory

[biowaffeln]

as someone who likes to keep my dotfiles inside ~/.config it's a bit annoying that the agent-memory feature hardcodes ~/.pi as the global memory dir. This PR adds behaviour for respecting the PI_CODING_AGENT_DIR dir. Let me know if you have any feedback!

Code written by Claude Fable 5. Here's it's PR description:

 │ fix(memory): honor PI_CODING_AGENT_DIR for user-scope memory
 │
 │ While setting up agent memory I noticed that the user scope writes to a hardcoded ~/.pi/agent-memory/, ignoring PI_CODING_AGENT_DIR. Everything else in the
 │ codebase (custom-agents, enabled-models, settings, index) already routes through getAgentDir() — memory.ts seems to be the one exception. For anyone who
 │ relocated their config dir, the first memory: user agent silently resurrects ~/.pi.
 │
 │ Changes
 │ - user scope resolves to <agentDir>/agent-memory/<name> (default ~/.pi/agent/agent-memory/)
 │ - Back-compat: if the legacy ~/.pi/agent-memory/<name> exists and the new location doesn't, it keeps being used, so existing memories aren't orphaned. Once
 │   the new location exists, it wins.
 │ - Symlinked legacy dirs are ignored, consistent with the existing symlink defenses in this file
 │ - Tests for resolution, fallback, precedence, and symlink rejection (with homedir mocked); README updated
 │
 │ One thing worth a maintainer's eye: I went with lazy fallback rather than auto-migration — moving files felt too invasive for this fix. Happy to add
 │ migration if you'd prefer that.
 │
 │ npm test (751 passing), biome check, tsc --noEmit all clean.