feat: add transactional SQLite event store

[karngyan]

Summary

Implements logical PR 2 from the Milestone 1 roadmap.

• add the cross-platform autophagy-store crate with bundled SQLite/FTS5
• apply ordered migrations transactionally and pin each applied migration by SHA-256
• validate AEP events before persistence and canonicalize timestamps to UTC
• atomically maintain source provenance, session rollups, artifacts, and search rows
• make identical reimports no-ops
• quarantine same-ID/different-content payloads without overwriting canonical evidence
• index only policy-processed structural fields and explicit redaction-approved free text
• cascade session deletion through events, conflicts, links, and FTS while retaining shared artifacts
• run the full mise quality gate on pull requests and pushes to main
• update the repository and database architecture documentation to match the implementation

Trust boundaries

• no network or cloud dependency at runtime
• raw event JSON is persisted for evidence but never blindly copied into FTS5
• raw tool input is stored for deterministic analysis but requires an explicit sanitized projection before search indexing
• foreign keys, secure deletion, busy timeout, and WAL mode are configured per connection
• malformed, provenance-conflicting, and sequence-conflicting inputs roll back atomically
• CI uses read-only permissions and actions pinned to immutable commit SHAs

Verification

• mise run check
• actionlint
• 8 storage/migration tests and 4 AEP contract tests
• disk close/reopen migration verification
• migration checksum drift and newer-schema rejection
• duplicate no-op and repeated conflict-quarantine assertions
• redaction-safe FTS and cascade deletion assertions
• strict Clippy with warnings denied
• rustdoc with warnings denied
• git diff --check