fix(mods): respect environment directory for loading mods

[KY64]

Summary

LETTA_MODS_DIR and LETTA_EXTENSIONS_DIR env vars were respected by install / update / remove commands (via
resolveDefaultGlobalModsDirectory()), but ignored by both the runtime mod engine and letta mods list. This meant:

1. letta install npm:foo → installed to the custom directory ✅
2. letta mods list → looked in ~/.letta/mods/ instead
3. Starting the TUI → loaded mods from ~/.letta/mods/ instead

Silent failure: the CLI appeared to work but the agent runtime never saw the mod.

Fixes #3333.

Root Cause

• src/mods/mod-engine.ts called getGlobalModsDirectory() and
  getLegacyGlobalExtensionsDirectory() which is resolved to the hardcoded defaults (~/.letta/mods/, ~/.letta/extensions/) even
  when env vars were set.

• src/cli/subcommands/mods.ts had the same pattern: listMods() called getGlobalModsDirectory() and
  getLegacyGlobalExtensionsDirectory() directly, bypassing env var resolution entirely.

Changes

src/mods/paths.ts

• New ResolvedGlobalModDirectories interface and resolveGlobalModDirectories() function — resolves both
  directories independently from env vars, falling back to the hardcoded defaults. Unlike
  resolveDefaultGlobalModsDirectory(), this does NOT fall back from ~/.letta/mods/ to ~/.letta/extensions/ — the
  runtime treats them as separate sources.

src/mods/mod-engine.ts

• resolveLocalModSources() now respect env vars for mods directory.
• getLegacyExtensionMigrationTarget() now resolves the migration target root from env vars instead of the
  hardcoded default, so the diagnostic message points to the correct custom directory.

src/cli/subcommands/mods.ts

• listMods() now respect env vars for mods directory.

[carenthomas]

This keeps an empty string from getModsDirectoryFromEnv because it uses ??. That regresses the existing env parsing where LETTA_MODS_DIR=/whitespace is treated as unset: runtime/list now resolve the global mods root to "" and stop loading the default mods dir. Can we normalize empty strings to undefined, or use the same || fallback here?

[KY64]

Right, let me update.

[KY64]

Updated a408d9c