fix: preserve invalid config before fallback

[0disoft]

Summary

• validate the minimum required config.json shape when loading opencodex config
• preserve invalid config files by copying them to config.json.invalid-<timestamp> before falling back to defaults
• warn the user once per process when the fallback path is used
• honor OPENCODEX_HOME for config/pid paths so config behavior can be tested without touching the real home directory

Fixes #25

Verification

• C:\Users\cherr\.bun-canary\bin\bun.exe test tests/config.test.ts
• C:\Users\cherr\.bun-canary\bin\bun.exe run typecheck
• C:\Users\cherr\.bun-canary\bin\bun.exe test tests

[lidge-jun]

Thanks for the original fix. I rebuilt the intent on top of the current dev branch instead of merging this stale branch directly, because current dev has since added multi-auth credential generation/tombstone/refresh-lock hardening and this PR now conflicts with those files.

Landed on dev as:

• 4d9d9f9 fix: preserve invalid config before fallback
• Co-authored-by: 0disoft <rodisoft1@gmail.com>

What was preserved from this PR:

• invalid config.json is copied to config.json.invalid-<timestamp> before fallback;
• fallback logs a warning with the config path and reason;
• warning/backup happens once per config path per process;
• regression tests cover invalid JSON and invalid config shape.

What changed for current dev compatibility:

• kept the existing dynamic OPENCODEX_HOME path helpers already present on dev;
• did not touch src/codex-account-store.ts, preserving current generation/tombstone/CAS refresh hardening;
• allowed existing port: 0 and providers: {} test/runtime contracts while still rejecting missing required shape and mismatched non-empty defaultProvider.

Verification:

• local bun run typecheck: pass
• local bun run privacy:scan: pass
• local bun test tests: 356 pass / 0 fail
• GitHub Cross-platform CI on dev run 28138885594: ubuntu + windows success

Closing this PR as superseded by the credited dev rebuild.