tweak Claude's allow and deny settings further

Allow git fetch, useful for fetching specific commits from Gerrit
such as those from unmerged patchsets.
Similarly, git apply can be useful when applying a diff or patch.

Allow fetching files and gists from GitHub, which is useful when
instructing the agent to investigate a user bug report with files.

Finally, I'm getting tired of reminding the agent to not cat and sed
to read and write files, which always requires permission,
even though I already placed this rule in CLAUDE.md.
Similarly, it insists on using `git -C /path/to/repo` and temporary
files in /tmp, which again interrupts sessions for no reason.

Try forbidding these in the settings; the agent tends to try the proper
mechanism once rejected manually, so I suppose it should also work
when rejected automatically via the settings.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I9c2db0ce00bf5bda4533658494bb5373a998642e
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1231231
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Matthew Sackman <matthew@cue.works>