#21521: fix: re-resolve skill paths at runtime for cross-machine portability

## Summary - When a session is created on machine A and later loaded on machine B, the cached `skillsSnapshot.prompt` contains absolute paths from machine A (e.g. `/Users/alice/...`) that are meaningless on machine B - Modified `resolveSkillsPromptForRun()` to re-generate the prompt from `resolvedSkills` at runtime using `compactSkillPaths()` against the current machine's home directory, instead of blindly returning the cached prompt - Legacy snapshots without `resolvedSkills` still fall back to the cached prompt for backward compatibility Fixes #21508 ## Test plan - [x] Existing e2e tests updated and passing (3 tests) - [x] New test verifies that a snapshot with a foreign home path produces correct `~/` paths on the current machine - [x] All `src/agents/skills/` unit tests passing (7 tests) - [ ] Manual: create a session on machine A, move state to machine B, verify skill paths resolve correctly 🤖 Generated with [Claude Code](https://claude.com/claude-code) <!-- greptile_comment --> <h3>Greptile Summary</h3> This PR fixes a cross-machine portability issue where skill paths cached in session snapshots from one machine (e.g., `/Users/alice/...`) were not updated when the session was loaded on a different machine (e.g., `/home/node/...`). The fix modifies `resolveSkillsPromptForRun()` to re-generate the prompt at runtime using `compactSkillPaths()` against the current machine's home directory when `resolvedSkills` is available, while maintaining backward compatibility by falling back to the cached prompt for legacy snapshots. Key changes: - Re-generates skill prompts at runtime from `resolvedSkills` instead of blindly returning cached prompt - Uses `compactSkillPaths()` to convert absolute paths to `~/` relative paths based on current machine's home directory - Preserves backward compatibility for legacy snapshots without `resolvedSkills` - Adds comprehensive test coverage for the portability fix <h3>Confidence Score: 4/5</h3> - This PR is safe to merge with low risk - The implementation is clean, well-tested, and maintains backward compatibility. The logic correctly re-generates prompts at runtime using the existing `compactSkillPaths()` helper, which has its own test coverage. The new test validates the portability fix scenario. The change is narrowly scoped to fix a specific portability issue without affecting other functionality. One point deducted only because tests couldn't be executed in this environment to verify they pass. - No files require special attention _{Last reviewed commit: e6126c7} <!-- greptile_other_comments_section --> _{(3/5) Reply to the agent's comments like "Can you suggest a fix for this @greptileai?" or ask follow-up questions!} <!-- /greptile_comment -->