refactor: deduplicate cross-platform stat and JSON parsing patterns

[artifex-bot]

What

• Added get_file_mtime(), get_file_size(), and get_file_mtime_human() helpers to lib/platform.sh to centralize cross-platform stat commands
• Updated handoff.sh, session-start.sh, and checkpoint.sh to use the shared helpers instead of inline stat fallback chains
• Reduced redundant jq invocations in lib/hook-config.sh by combining the type-check and value-extraction into a single jq call per config file
• Deduplicated JSON parsing fallback pattern in tldr-stats.sh and track-tldr.sh with a shared json_num() helper
• Simplified logging setup in compile-skills.sh to a single conditional pattern

Why

The codebase had the same cross-platform stat fallback pattern (stat -f%m ... || stat -c%Y ... || echo 0) duplicated across 5+ scripts. Similarly, get_hook_config() was making 2 jq calls per config file (type check + value extraction) when 1 suffices. These are low-effort, high-impact maintainability wins that reduce code duplication and improve consistency.

[arzafran]

Closing as obsolete.

All 8 files this PR touches were removed in the bash → TypeScript-on-Bun migration (Phase 7, April 2026 — see docs/migration-coexistence.md and current CLAUDE.md):

Old bash	Current TS equivalent
lib/platform.sh	src/lib/platform.ts
lib/hook-config.sh	src/schemas/hooks.ts + hook wiring in src/setup.ts
scripts/handoff.sh	src/scripts/handoff.ts
scripts/session-start.sh	src/hooks/session-start.ts
scripts/checkpoint.sh	src/scripts/checkpoint.ts
scripts/tldr-stats.sh	src/scripts/tldr-stats.ts
scripts/track-tldr.sh	src/scripts/track-tldr.ts
scripts/compile-skills.sh	src/scripts/compile-skills.ts

The duplication this PR addresses (cross-platform stat fallbacks, double jq calls for type + value extraction) doesn't apply to the TS ports — they use node:fs.stat and typed JSON parsing directly via zod.

Thanks for the refactor pass — if you want to take a look at consolidation opportunities in the current TS codebase, that would be welcome.