Trim repo-derivable content from CLAUDE.md

[folbricht]

CLAUDE.md is loaded into context at the start of every session, so anything a session can reconstruct by reading the repo is dead weight it pays for each time. This drops the parts that qualify, taking the file from 5627 to 1117 characters.

Removed:

• Project overview — a paraphrase of README.md and doc.go.
• Build and Test Commands — the standard go build/test/install/fmt/mod tidy invocations. go fmt, go build and go mod tidy are already enforced mechanically by .pre-commit-config.yaml, and the CI line restates .github/workflows/validate.yaml. The one non-standard rule here, building into cmd/desync/, survives under Code Style where it is stated with its rationale.
• Architecture — interface signatures copied from store.go, the store implementation and seed file listings, the data pipeline tour, the CLI command list, and the config/env-var inventory.
• Three generic Key Patterns bullets — interface-driven composition, lazy evaluation, context-based cancellation: descriptions of what the code already shows.
• Module — the module path from go.mod, which had also gone stale: it claimed Go 1.24.0 while go.mod says 1.25.0. Derivable content goes out of date silently, which is the second reason to drop it.

Kept — the content the code cannot teach on its own:

• The t.Fatal()-in-goroutines restriction and the avoid-recompression rule, both with their PR references (Do not call t.Fatal() from non-main goroutines #291, Do not recompress chunks if we already have the compressed form #289).
• The testify and build-output conventions, which differ from the Go defaults and so would be learned wrong from the code alone.

No code changes.