sec (6/9): run user-defined scanners in an isolated working directory

[jesse-merhi]

What changes

A user-defined scanner no longer runs with its working directory set to the
target's folder. On a host run it now runs in a fresh isolated temp directory
(clawscan-scanner-*, removed after the run); under Docker the workdir stays
empty so nothing target-adjacent is mounted.

Why it matters

Running with cwd = the target's directory hands the scanner writable access
to every file sitting next to the target. A malicious or buggy scanner could
modify or drop files beside the skill being scanned (and under Docker that
directory would be a writable bind mount). Giving the scanner a private throwaway
cwd removes that ambient write access; the target is still passed explicitly as
an argument.

Before / after (runnable)

Scanner that records its cwd and tries to write a neighbor file:

command: "echo {{target}} >/dev/null; pwd 1>&2; touch NEIGHBOR_LEAK; echo '{}'; exit 9"

	Behavior
Before	cwd = target dir; NEIGHBOR_LEAK lands next to the skill
After	cwd = isolated temp dir; nothing left beside the target

$ clawscan ./skill --config cwd.yml --profile demo --sandbox off --json
# scanner error stderr shows its cwd:
exit status 9: /private/var/folders/.../T/clawscan-scanner-2472157201
# and next to the target:
marker next to target? no — ran in isolated temp dir, cleaned up

Verify

go test ./internal/runner/ -run 'IsolatedCwd' -count=1

TestUserDefinedScannerUsesIsolatedCwd asserts the host run's cwd is a fresh
temp dir (not the target dir) and that Docker runs use an empty workdir.

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I am starting a fresh review of this pull request: sec (6/9): run user-defined scanners in an isolated working directory This is item 1/1 in the current shard. Shard 0/1.

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