add --json output to the query commands

Scripts and health checks had to scrape formatted text, which changes whenever
the display does.

--json is global and applies to status, ls, pins, sync list, devices list,
transfers, activity, and cache inspect. Action commands keep their human
output; a script that needs to know whether one worked has the exit code.

Two decisions worth recording:

The daemon's own reply is serialized rather than a hand-built parallel
structure, so new fields reach the JSON without anyone remembering to add them
-- but the variant tag is stripped first. Serializing CtlResponse directly
yields {"SyncFolders": {"items": [...]}}, which would make every caller write
jq '.SyncFolders.items[]' and couple it to the name of an internal enum
variant. Callers get {"items": [...]}.

A daemon-side error prints its payload and then fails the command. The first
cut exited 0 after printing the error body, which was caught by running
`pdfs --json ls NoSuchFolder`: a script would have read a failed lookup as
success. The body is still emitted so `kind` is available, but the exit code
is what `set -e` and `if pdfs ...` actually test.

status composes its object by hand instead, being the one query that merges
the local keyring with the daemon -- including the logged-in-but-no-daemon
case, which has no daemon reply to pass through at all.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JFzNQyb4TWrCRWcnegqWxY

README.md

@@ -92,6 +92,30 @@ For the local metadata database and content cache:
 the database while it runs, so it is a deliberate operation rather than
 something the daemon does on a timer.
 
+## Scripting (`--json`)
+
+Query commands accept a global `--json` flag and emit machine-readable output:
+
+```console
+$ pdfs --json sync list | jq '.items[] | select(.state != "idle") | .local_path'
+$ pdfs --json cache inspect | jq '.db_reclaimable_bytes'
+$ pdfs --json status | jq -r '.mount.mountpoint'
+```
+
+Supported on `status`, `ls`, `pins`, `sync list`, `devices list`, `transfers`,
+`activity`, and `cache inspect`. Commands that perform an action keep their
+human output — a script that needs to know whether one worked has the exit code.
+
+Two things worth relying on:
+
+- **The payload is unwrapped.** Output is `{"items": […]}`, not the daemon's
+  internal `{"SyncFolders": {"items": […]}}`, so scripts never name an internal
+  variant.
+- **Errors still fail.** A daemon-side error prints its JSON body (with a
+  machine-readable `kind`) on stdout *and* exits non-zero, so `set -e` and
+  `if pdfs …` behave as expected rather than treating a failed lookup as
+  success.
+
 ## Performance & Caching
 
 The client includes several optimizations designed for high efficiency, a low memory footprint, and a responsive user experience: