[shovon]

Closes #63.

What changed

Code (packages/ledger/ledger.ts):

• NAMED_EXTRAS is the list of extras ledger: ignore what the format doesn't name, instead of refusing or reporting it #104 left empty, and AGENTS.md is its first entry, with the exact bytes every build of format 1 writes. The text says the folder is a ledger written by Pb Notes, points to https://github.com/shovon/pbnotes for the format, and says not to modify, rename or delete anything by hand.
• openOrCreate writes each named extra after the marker with an exclusive create (createOnce, factored out of createMarker). One that is already there is left alone, whatever it holds.
• inspect counts a folder that holds only named extras as empty, so a sync that delivers AGENTS.md before LEDGER still lets the folder open. #scan does not list them in status.ignored.
• Only a file counts. A folder named AGENTS.md is read as a writer, like any folder at the top.

Spec (packages/ledger/multi-writer.md): AGENTS.md is listed under "Named extras" and in the layout, the opening table and the "counts as empty" rule mention named extras, and the tests section covers the new test. app/CLAUDE.md says the same.

For the reviewer

• Backfill, for now. The file is written on every open, not only on creation, so ledgers made before this get one too. The cost is that a user who deletes it gets it back on the next open. ledger: stop backfilling AGENTS.md into existing ledgers #106 limits it to creation.
• Best effort. The writes go through Promise.allSettled, so a folder that cannot take the file (read-only, full) still opens. The ledger is whole without it. The cost is that a failed write is silent.
• The text is now fixed. Under ledger: ignore what the format doesn't name, instead of refusing or reporting it #104's rule for files that several writers may create, every build of one format version writes the same bytes. Changing the wording means a new format version.
• openWriter does not write it. Its test says it puts nothing in the folder but its own directory, and that stays true.

Tests

• New test in ledger.test.ts: a new ledger gets the exact bytes; an existing ledger without one gets one; one already there is not overwritten and is not in status.ignored; a folder holding only AGENTS.md inspects as missing and opens; a folder named AGENTS.md holding a segment is folded as a writer.
• Three tests that listed the top of a folder now expect AGENTS.md. The openWriter one compares against the folder as it was before openWriter ran, so it still checks that openWriter adds only its own directory.
• npm test (all workspaces), npm run typecheck and the app's lint pass.

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