ci: split inspect and sync lanes in depot ci

[anandpant]

Summary

• add a changed-path classifier job to Depot CI
• split Rust verification into inspect/core lanes and sync/export lanes
• avoid rebuilding the heavy S3/AWS/Arrow stack on clearly unrelated PRs

What Changed

• add Changed Paths job
• split CI into:
  • Clippy / Inspect
  • Test / Inspect
  • Clippy / Sync
  • Test / Sync
• gate inspect lanes on:
  • apps/convex-inspect/**
  • crates/convex-sync-core/**
  • shared workspace files
• gate sync lanes on:
  • apps/convex-sync/**
  • crates/convex-export-s3/**
  • shared workspace files
• update the top-level README verification section to match the new CI shape

Why

The binary split is already merged. This PR is the operational payoff:

• light/source-only work can finish sooner
• heavy sync/export work remains isolated in its own lanes

Verification

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace
• actionlint -config-file .github/actionlint.yaml .github/workflows/*.yml
• live Depot CI proof against this branch:
  • run mbt4ztm544
  • confirmed jobs fan out as:
    • Changed Paths
    • Clippy / Inspect
    • Test / Inspect
    • Clippy / Sync
    • Test / Sync

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