fix(sessions): deduplicate Claude reasoning text

[ScriptedAlchemy]

Root cause

#372 emitted canonical Claude kind=reasoning rows but kept the same plaintext thinking blocks in the searchable kind=message blob. That produced duplicate hits and retained the JSON search pollution the structured row was meant to remove.

Fix

Strip thinking and redacted_thinking blocks from the indexed assistant message row. Preserve visible text and tool-use blocks, keep plaintext thinking in the linked reasoning row, and leave the source transcript lossless on disk.

Tests

• Claude parser unit tests: 8 passed
• Claude transcript ingest: 17 passed
• Structured backfill regression: passed
• cargo fmt --all -- --check
• git diff --check

Cargo verification used /fast/cargo-target/tracedecay-claude-reasoning-dedup.

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