feat(threads): add durable session thread stores

[serrrfirat]

Summary

• add feature-gated libSQL/PostgreSQL SessionThreadService implementations for canonical Reborn threads/transcripts
• persist canonical thread records, messages, summary artifacts, and inbound idempotency rows with transactional mutations and payload/column integrity validation
• cover durable parity for history/context/redaction, cross-scope denial, assistant draft idempotency, concurrent monotonic sequence assignment, and long external event IDs

Refs #3204

Verification

• CARGO_TARGET_DIR=/tmp/ironclaw-cargo-target-threads IRONCLAW_SKIP_POSTGRES_TESTS=1 cargo test -p ironclaw_threads --all-features
• CARGO_TARGET_DIR=/tmp/ironclaw-cargo-target-threads cargo test -p ironclaw_threads
• CARGO_TARGET_DIR=/tmp/ironclaw-cargo-target-threads cargo clippy -p ironclaw_threads --all-targets --all-features -- -D warnings
• cargo fmt --check --package ironclaw_threads
• git diff --check
• CARGO_TARGET_DIR=/tmp/ironclaw-cargo-target-threads cargo test -p ironclaw_architecture reborn_crate_dependency_boundaries_hold

Postgres tests use the existing explicit skip convention when no live DB is configured. Set IRONCLAW_THREADS_POSTGRES_URL or DATABASE_URL to exercise a live backend.

Review

• Reviewer pass after fixes: no Critical/High/Medium findings.

[gemini-code-assist]

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