refactor(adapters): extract shared json and path helpers

[samzong]

What's changed?

• Add json_util.rs with shared jsonl_indexed, rfc3339_ms, and json_i64 helpers
• Add paths.rs with resolve_home_dir for consistent ~/… directory resolution
• Add first_timestamp / last_timestamp in mod.rs for session started_at/updated_at fallback chains
• Migrate antigravity, claude_code, cline, codex, copilot, cursor, gemini, grok, and pi adapters to the shared helpers
• Document the new helpers in src/adapters/AGENTS.md

Why

• Remove duplicated JSONL parsing loops, RFC3339 timestamp coercion, home-dir resolution, and timestamp fallback logic across adapters
• Centralize coercion behavior (e.g. json_i64 uses i64::try_from for u64) so fixes apply consistently

Verification

• cargo test (core recall crate): 333 tests passed
• cargo clippy --workspace --all-targets -- -D warnings: clean
• cargo fmt --all -- --check: clean