Harden remote checks and migrate automation to just

[jleni]

Summary

• harden daemon remote existence probes with a short warming wait, a HEAD-probe circuit breaker, and more authoritative key-cache negatives
• improve S3 HEAD diagnostics so backend-specific failures include object path, error metadata, and request IDs
• replace Make with Just, install just in CI via mise, and document just check / just ci as the repo task contract

Architecture

This PR intentionally excludes the kache-core / kache-service refactor work. It is scoped to release hygiene only:

• runtime hardening in the existing daemon path
• repo and CI ergonomics for contributors

The daemon changes keep HEAD as a fallback path instead of a noisy steady-state path. The automation changes make Justfile the single task interface while leaving Rust toolchain setup in CI on dtolnay/rust-toolchain to keep the migration low-impact.

Verification

• just --list
• just --dry-run ci
• cargo fmt -- --check
• cargo clippy -- -D warnings
• cargo test
• rg -n make README.md CONTRIBUTING.md .github/workflows Justfile mise.toml