logs: Add --log-to-file to write debug logs to a file

[brian-kubisiak-skydio]

Verbose output is usually wanted after a command has already run and
exited, which -v can't provide retroactively. Leaving -v on all the
time isn't a workable substitute either, since it clutters normal
output.

Add a second logging backend, off by default and enabled with
--log-to-file. It writes debug logs to $XDG_STATE_HOME/revup/logs, one
file per invocation named after the start time and pid, keeping 20
runs of history. The root logger level now only gates what any handler
can see; each handler decides for itself, so the console keeps showing
INFO while the file gets DEBUG. Both handlers share one filter, so an
oauth discovered partway through a run is redacted in the file too.

Co-Authored-By: Claude Opus 5 noreply@anthropic.com
Signed-off-by: Brian Kubisiak brian.kubisiak@skydio.com

[brian-kubisiak-skydio]

Reviews in this chain:
└#300 logs: Add --log-to-file to write debug logs to a file

[brian-kubisiak-skydio]

#	head	base	diff	date	summary
0	6ffa1e71	20c37803	diff	Sep 15 15:04 PM	5 files changed, 340 insertions(+), 30 deletions(-)
1	54f5427b	20c37803	diff	Sep 15 15:36 PM	1 file changed, 1 insertion(+), 3 deletions(-)
2	c2bd5b4f	20c37803	diff	Sep 16 13:38 PM	2 files changed, 20 insertions(+), 6 deletions(-)

[aaron-skydio]

Nice, yeah this is useful