INTR 451 - Remove hardlinks from pack publishing and tool

[FarinM]

Remove hardlinks from pack publishing and tool materialization

Motivation

Hardlinks are not part of the filesystem surface we expect browser-oriented storage backends to provide. Removing this requirement narrows the filesystem API needed by these workflows and unblocks the broader work to make the storage layer runtime-agnostic.

Summary

Removes the two remaining hardlink requirements:

• Publishes completed Git packs using atomic renames.
• Copies tool-package files from the extraction cache into deployment layouts.

Pack publishing now reserves transfer IDs through exclusive staging-directory creation, preventing concurrent publishers from overwriting each other. Partial publication failures clean up the staged files and unindexed pack where possible.

The tool-package cache still avoids downloading and extracting packages repeatedly, but deployment layouts now contain independent file copies. This increases disk usage and write I/O in exchange for requiring fewer filesystem capabilities.

This is a prerequisite for making the storage layer runtime-agnostic; it does not introduce the pluggable filesystem API itself.

Testing

• Added coverage for pack rename ordering, existing destinations, concurrent transfer IDs, and partial publication cleanup.
• Added coverage confirming materialized tool files are independent from the cached extraction.
• make all passes:
  • 6,494 unit/package tests
  • 753 integration tests
  • 2 expected live tests skipped

[linear]

INTR-451

[alexanderguy]

LGTM