#7569: fix: add null checks to prevent TypeError in subagent spawn

Add null coalescing operators to prevent 'Cannot read properties of undefined (reading trim)' errors when spawning subagents: - lanes.ts: Guard key.trim() with ?? "" - system-prompt.ts: Guard file.path.trim() with ?? "" These crashes occur when optional string parameters are undefined, causing subagents to fail immediately on spawn. Fixes #7443 <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR adds defensive nullish-coalescing guards around `.trim()` calls that can receive `undefined` at runtime, preventing subagent spawn failures (notably when normalizing session lane keys and when scanning injected context file paths for `soul.md`). The change is localized to `resolveSessionLane` in `src/agents/pi-embedded-runner/lanes.ts` and the `hasSoulFile` detection logic inside `buildAgentSystemPrompt` in `src/agents/system-prompt.ts`. <h3>Confidence Score: 4/5</h3> - This PR is low-risk and primarily adds defensive guards to prevent runtime crashes. - Changes are small and localized, but there may be a minor type-safety concern where a function typed to accept `string` is now handling `undefined` at runtime (suggesting an upstream typing mismatch). - src/agents/pi-embedded-runner/lanes.ts (type signature vs runtime inputs) <!-- greptile_other_comments_section --> <!-- /greptile_comment -->