Support inserting tree entries that reference arbitrary existing objects

[joeycarpinelli]

Problem

Ledger::create always writes new blobs for field values. There is no way to insert a tree entry whose OID points to an existing git object (commit, tree, or blob) with the correct file mode.

Use case

Forge reviews keep an objects/ subtree that pins target objects for GC:

objects/
├── <commit-oid>   # mode 160000 (gitlink)
├── <blob-oid>     # mode 100644
└── <tree-oid>     # mode 040000

Each entry's OID must point to the actual object, not a new blob containing placeholder content. Currently, callers must work around this with a fixup_pin_entries post-pass that:

1. Reads back the commit that Ledger::create just wrote.
2. Rebuilds the objects/ subtree with the correct OIDs and file modes.
3. Creates a second commit and force-updates the ref.

This doubles the number of commits per creation and is fragile.

Proposed API

A new Mutation variant (or a new field type) that tells the ledger to insert this entry name with this existing OID and file mode:

// Strawman — exact API shape is up to you.
Mutation::Pin("objects/abc123...", oid, FileMode::Commit)

The ledger would insert the tree entry directly, the same way treebuilder.insert(name, oid, mode) works, instead of creating a new blob.

Alternatives considered

• Separate subtree + manual commit: what we do today. Works, but the two-commit dance is ugly and makes ref history harder to follow.
• Store OID strings as blob content: keeps one commit but defeats the purpose — GC can still collect the referenced objects because nothing in the tree actually references them.

Generated-by: Claude Code (Claude Opus 4.6)