feat: add last-write-wins workspace policy

[kauandotnet]

Summary

• add an explicit write permission and mode: "overwrite" Workspace operation that delegates to the ordinary Files SDK upload path
• add last-write-wins copy through bounded download plus upload, and unconditional delete guarded by both write and delete
• add bounded binary readBytes() and a selectable AI tool mutation policy while keeping conditional mode as the default
• retain every existing native conditional create, replace, copy, move, and delete path and the fail-closed Files policy guard from fix: fail conditional operations closed with Files policy #30

Safety decisions

• last-write-wins move is deliberately not exposed: download/upload/delete could delete a newer source generation
• overwrite options reject ignored ETags and malformed/legacy modes before provider I/O
• unconditional delete requires a new write grant in addition to the existing delete permission, so current delete-only mounts do not gain weaker authority
• copy uses the ordinary download and upload pipeline instead of provider-side copy, preserving transforms such as decrypt/re-encrypt and enforcing workspace byte limits
• the bundled Files SDK encryption() plugin is tested only as pipeline compatibility evidence; application-specific key custody, strict encrypted-only reads, AAD, and copy/move policy remain application-owned
• no Files SDK version bump, low-level conditional API change, R2 change, package publication, or change to release PR chore: release @nestm/storage (alpha) #31

Verification

• pnpm test — 255 unit tests; 55 e2e passed and 4 live tests skipped
• pnpm check
• pnpm verify:pack — build, strict publint, tarball install, packed consumer test
• independent API/security diff review

Tracks #29. Native CAS can return to Workspace Files after haydenbleasel/files-sdk#138 / haydenbleasel/files-sdk#139 (or an equivalent upstream release) routes conditional operations through the same plugin, hook, and receipt boundary.