perf(cli): stop session logs growing by tens of MB a day

[chphch]

Two writers dominate a Happy CLI session log, and neither carries anything a reader uses. logToFile inspects every non-string argument at depth: 5, so a single logger.debug('...', err) on an error that transitively holds a Node socket — which every failed HTTP or socket.io call produces — expands into hundreds of lines of TLS cipher lists and internal symbols. Separately, sessionScanner writes the path it is about to read plus a scan summary on every 3-second poll, for the entire life of a session, which is ~2400 identical lines an hour whether or not anything changed. On my machine ~/.happy/logs had reached 1.3 GB, with individual week-old sessions holding 80–90 MB each; measured across those files, the object dumps were 55% of all bytes written and the scanner's repeated lines another 32%.

This does not add a log level or a config knob — it stops writing bytes nobody reads:

• Cap one inspected argument at 4000 chars. That keeps an error's own top-level fields, which is the part anyone actually reads, and drops the object graph hanging off them. Small objects are untouched.
• Log scanner state changes, not polls. An absent transcript is reported once per absence rather than once per tick, and the scan summary is written when the poll actually moved a message, otherwise at most once a minute so "the scanner is alive and reading N entries" stays observable.

Proof

No UI surface, so this is measured rather than filmed.

Steady-state growth, two idle sessions on the same machine over the same 180 s window — one still on the pre-patch binary, one on the patched build:

session	log growth	per day
pre-patch	356 KB/h	8.3 MB
patched	16 KB/h	0.4 MB

The patched session's log shows the summary line at 60-second spacing (08:06:34, 08:07:34, 08:08:35) where the pre-patch one still writes a Reading session file: plus a summary every 3 s.

Truncation, against a real object of the kind that filled the logs (a live https response, which holds a TLS socket):

depth-5 inspect : 24,457 chars, 697 lines
after the cap   :  4,030 chars, 139 lines   + "[truncated 20457 more chars]"
small object    :     40 chars — unchanged, no marker

packages/happy-cli tsc --noEmit is clean and the sessionScanner + startFileWatcher suites pass. I have been running this on my own daily-driver instance since building it, which is where the 180 s comparison above was taken.

The two halves are independent and I am happy to split them into separate PRs if you would rather review them apart — they are together here because they were found by the same measurement of the same file.