Split provider.rs (3,978 lines) and core lib.rs (2,402 lines) — documented merge-hazard hot files

[haasonsaas]

Problem

Three files have grown large enough to be both hard to navigate and, per AGENTS.md, an active merge hazard:

• crates/sandboxwich-worker/src/provider.rs — 3,978 lines (tests were already extracted into provider/tests.rs in a prior pass, so this is all implementation).
• crates/sandboxwich-core/src/lib.rs — 2,402 lines of shared typed contracts.
• crates/sandboxwich-worker/src/main.rs — 2,110 lines.

AGENTS.md's "Landing PRs" section already names crates/sandboxwich-worker/src/provider.rs as one of the shared hot files that produced semantic merge conflicts during the 2026-07-09 audit wave (three incidents: #96×#98, #97×#99, #97/#99×#100) and instructs auditing all call sites after merging anything that touches it. The file's size is a direct contributor to why merges collide there.

Evidence

• crates/sandboxwich-worker/src/provider.rs: 3,978 lines (wc -l), named explicitly in AGENTS.md under "Landing PRs" as a shared hot file requiring post-merge call-site audits.
• crates/sandboxwich-core/src/lib.rs: 2,402 lines (wc -l).
• crates/sandboxwich-worker/src/main.rs: 2,110 lines (wc -l).
• Precedent: crates/sandboxwich-api/src/main.rs was already split this way in Split sandboxwich-api main.rs and http_contract.rs into modules #110 into handlers/ by resource family, db.rs, auth.rs, scheduler.rs, cleanup.rs, etc., per AGENTS.md's "Layout" section.

Suggested approach

Apply the same #110-style split used for the API crate:

• provider.rs: separate by concern, e.g. Kubernetes manifest construction (Pod/PVC/Service/NetworkPolicy/VolumeSnapshot builders), the dry-run vs apply-mode branching, reconciliation, and the kubectl-invocation plumbing (run_kubectl_command_with_stdin and friends) into their own modules under provider/.
• crates/sandboxwich-core/src/lib.rs: split typed request/response/event contracts by resource family (sandboxes, jobs/leases, snapshots, workers, runtime resources) into submodules, re-exported from lib.rs.
• crates/sandboxwich-worker/src/main.rs: separate the CLI arg parsing/entrypoint from the work loop (work_loop, handle_lease, claim, heartbeat/registration/drain helpers), mirroring the "thin entrypoint, logic in modules" convention AGENTS.md already establishes for the API crate.

Smaller, concern-scoped modules reduce the odds of two PRs touching the same lines and reduce the blast radius AGENTS.md asks reviewers to audit after every merge.