Add unit tests for sessions parser

[abachman]

Summary

• Add sessions_test.go with table-driven tests for oneLine, truncateRunes, summarizeToolInput, and normalizeRecord
• Add testdata fixtures (simple.jsonl, tool_use.jsonl, empty.jsonl) for integration-style ParseSessionFile tests
• Add coverage Makefile target matching CI flags (-race, -coverpkg, -covermode=atomic)

Test plan

• go test ./internal/claude/... — all 20 tests pass
• make coverage — runs with race detector and generates coverage.txt

🤖 Generated with Claude Code

[github-actions]

⚠️ Note: Baseline coverage from main branch is not available (artifact may be expired). Showing current coverage for changed files only.

Merging this branch will not change overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/reasonable-systems/ccsessions/internal/claude	0.00% (ø)

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Coverage by file

Changed unit test files

• github.com/reasonable-systems/ccsessions/internal/claude/sessions_test.go