Add .github/copilot-instructions.md

[Copilot]

Documents codebase conventions and architecture for GitHub Copilot (and human contributors) in the canonical .github/copilot-instructions.md location.

What's covered

• Workspace layout — table of all 9 crates and their roles
• Rust conventions — toolchain (1.85.0), edition (2021), cargo fmt / cargo clippy -D warnings, mandatory Apache 2.0 file header
• Error handling — DFError enum, Result<T> alias, OrErr trait; no unwrap() in production paths
• Logging & tracing — tracing crate usage, log-level guide, #[instrument(skip_all)], span propagation into spawned tasks
• Async conventions — Tokio patterns, Arc<T> sharing, RwLock preference, spawn_blocking for CPU work
• Testing — #[cfg(test)] placement, #[tokio::test], test dependencies (mocktail, wiremock, criterion), test commands
• Key dependencies — reference table (tonic, hyper, reqwest, rocksdb, opendal, quinn, etc.)
• Storage layer — three-layer design (RocksDB metadata → content files → LRU cache), DatabaseObject trait contract
• Build & CI — canonical cargo check / fmt / clippy / llvm-cov commands and workflow descriptions
• Platform targets — four supported triples, Linux-only feature gating pattern
• Performance notes — JeMalloc, release profile flags, pprof integration

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[gaius-qi]

LGTM

[chlins]

lgtm

[gaius-qi]

LGTM