Fix configured memory path normalization

[chandra447]

Summary

Fixes the memoryDir: "~/.pi/agent/pi-hermes-memory" config bug from #42 by normalizing configured storage paths before startup uses them.

What changed

• expand and normalize memoryDir during config loading so ~ resolves to the user home directory instead of becoming a literal repo-local path
• resolve relative memoryDir values against ~/.pi/agent so config behavior is stable and not dependent on the current working directory
• centralize path normalization in a new helper module for extension storage paths
• keep projectsMemoryDir constrained to a single safe directory segment under ~/.pi/agent, which matches the existing config contract and avoids opening a partially supported nested-path mode
• make startup migration and Markdown-to-SQLite sync use the Pi agent root explicitly instead of inferring it from memoryDir, so custom global memory roots do not break legacy project migration or project memory discovery
• add regression tests for ~ expansion, relative path normalization, unsafe projectsMemoryDir rejection, and the custom-global-root migration/sync behavior

Verification

• npm run check
• npx tsx --test tests/config.test.ts
• npx tsx --test tests/project-memory-migration.test.ts
• independent subagent review of the uncommitted diff, followed by a follow-up fix for the only finding around nested projectsMemoryDir handling

Notes

• Full npm test is still blocked in this environment by the existing better-sqlite3 vs Node ABI mismatch in SQLite-backed tests; this branch does not introduce a new path-related test failure there.

Fixes #42