fix(cli): resume selected session from cn ls

[JSap0914]

Summary

• pass the selected cn ls session id through to chat --resume
• add loadResumeSession() so targeted resume sets the current session before future saves
• remove the stale env-var session override path from ls

Fixes #12927

Verification

• npm test -- src/commands/ls.test.ts src/session.test.ts (38 tests passed)
• npm run typecheck currently fails before this change due missing local workspace package builds/type declarations such as @continuedev/config-yaml and existing core NodeNext import diagnostics in this checkout.

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