Add context migration for version upgrades

[th3nate]

Summary

• Adds automatic context migration that preserves accumulated project knowledge (patterns, decisions, learnings, verification evidence) when STATE_VERSION bumps (e.g., v10 → v11)
• Runs on first SessionStart after upgrade, merging content from all previous version directories (including legacy root-level files) into the new version
• Idempotent via .migrated marker file; section-aware merge with bullet-level dedup; atomic writes with .pre-migration.bak backups

Changes

File	Change
plugins/cc10x/scripts/cc10x_hooklib.py	Added parse_markdown_sections(), extract_bullets(), normalize_bullet() utilities
plugins/cc10x/scripts/cc10x_context_migration.py	New migration script — discovery, merge, dedup, marker, logging
plugins/cc10x/hooks/hooks.json	Added startup-only SessionStart hook before existing context injection

Merge strategy per file

• patterns.md — full section merge (all 10 contract headings)
• activeContext.md — selective (Decisions + Learnings only; ephemeral sections skipped)
• progress.md — selective (Completed + Verification only)

Test plan

• Unit tests for parse_markdown_sections, extract_bullets, normalize_bullet
• End-to-end migration test with legacy + v9 sources merging into v10
• Dedup verification (overlapping bullets across sources merged once)
• Idempotency verification (re-run finds 0 pending sources)
• Fresh install test (no sources = clean no-op exit)
• Hook contract test (stdin JSON, exit 0, valid stdout)

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