Add compaction source pruning

[khaliqgant]

Summary

• add trail compact --discard-sources to remove source trajectory JSON/Markdown/trace files and prune index entries after compaction
• expose source pruning through compactWorkflow and SDK auto-compaction options
• update the trail snippet, README guidance, and package formatting so lint passes

Verification

• npm run lint
• npm run typecheck
• npm test -- --run

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Guard missing index maps during source discard

Handle structurally invalid index.json the same way as markTrajectoriesAsCompacted(): in discardSourceTrajectories(), index.trajectories is dereferenced without a guard, so a valid-but-corrupt index (for example { "version": 1 } from manual edits or partial writes in any search path) throws after the compaction artifact has already been written. That turns trail compact --discard-sources into a hard failure and can leave users with partial side effects instead of the intended best-effort cleanup.

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[khaliqgant]

Fixed in c6ba4cc by validating the parsed index shape before touching index.trajectories, so structurally invalid indexes are skipped during best-effort source discard.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.