Managed refs and repository discovery: packed-refs, worktrees, shallow (Phase B.2)

[arturcic]

Loose refs, packed-refs, symbolic refs and HEAD resolution with glob enumeration; repository discovery (walk-up, .git file indirection, commondir split for worktrees, bare detection); .git/shallow parsing; detection of unsupported formats (SHA-256 object format, reftable) with clear errors.

Part of #5031 — see docs/design/managed-git-migration.md for the full design.

[arturcic]

Done via #5045 (merged into feature/managed-git).