[snowsword20]

Closes the proposal in issue #118 (headless scan mode for scripts and coding agents).

What

A scan subcommand on disktree-scan, the GPUI-free helper:

disktree-scan scan PATH [--apparent-size] [--no-hidden] [--min-size N]

It runs the same read-only scan the treemap draws and writes JSON Lines to stdout — one directory per line, parent before children, children largest first:

{"path":"E:\\aaprojects","bytes":33676751920,"files":364554,"dirs":69384,"modified":1793212365,"read_error":false}
{"path":"E:\\aaprojects\\githubrepo","bytes":27952605184,"files":298120,"dirs":61072,"modified":1793300000,"read_error":false}

--min-size N prunes printed directories while totals on surviving lines stay complete. Same promises as --stdio-v1: no stdin, no deletion surface, no GPUI dependency.

Why

Scripts and coding agents cannot open a window, and du is an order of magnitude slower than the scanner on the same tree (measured: >20 min vs 30 s cold / 2.6 s warm on a 364k-file HDD tree). The app already exports agent_prompt for the cleanup half; this gives agents the scan half, so the whole loop can run headless.

How

disktree_core::scan::scan is reused unchanged; the mode parses its own flags, calls it, and walks the returned tree writing one JSON object per line. ~180 lines in the helper, one new dependency (serde_json), no app or protocol changes.

Tests

Four new tests in the helper: root summary first, --min-size pruning (pruned subtree still counted on the parent), apparent vs allocated bytes, and flag validation (missing/repeated path, unknown option).

Related prior art (and why this still exists)

Three open proposals overlap with this one, all worth reading before judging it:

• disktree-cli: the scan, findings and removal guards without a window #32 disktree-cli — the ambitious shape: scan, findings, git state, and removal through disktree's own guards. It is the fuller product; it also carries a destructive surface and a new distribution artifact, which may be part of why it is still open.
• du for agents: a cached, GNU-compatible front end on disktree-core (building on #32) #47 — a GNU du personality on disktree-core, meeting agents where they already are (du -sh *).
• disktree --cli: print the tree to stdout, for agents and headless sessions #74 — a human- and agent-readable text tree from the main binary, keeping the JSON surface JSON.

This PR deliberately takes the smallest slice: JSON Lines out of the existing read-only helper. No new binary, no destructive surface, no opinion about what a fuller CLI should eventually be. If the direction lands on #32 or a --cli flag instead, this is ~180 lines to rebase or close — and the tests travel.