fix(amass): replace hardcoded /tmp/amass with a private per-task scratch dir

[Subramaniyajothi6]

Summary

Closes #1803.

The amass plugin's command_template hardcoded -dir /tmp/amass. That path is
world-writable and predictable, which allows symlink/hijack attacks and cross-user
collisions, and it is never cleaned up between runs.

Changes

• plugins/amass/metadata.json — replace the hardcoded /tmp/amass with the
  reserved token %SECUSCAN_SCRATCH_DIR% (checksum refreshed). The token contains no
  {...}, so it passes plugin validation and command-template interpolation through
  untouched as an opaque literal.
• backend/secuscan/executor.py — add a small, reusable per-task scratch-dir
  primitive:
  • SCRATCH_DIR_PLACEHOLDER constant + _substitute_scratch_dir() helper, which
    creates a fresh tempfile.mkdtemp(prefix="secuscan-<plugin>-") directory (mode
    0700, unpredictable name) and substitutes every placeholder token with its path.
  • _execute_standard_scanner() resolves the placeholder right after building the
    command and wraps command execution in try/finally so the directory is removed
    when the scan ends — on success, egress-validation failure, docker error, timeout,
    or crash.

Any plugin that needs a private, per-task working directory can now use the same
reserved token instead of a hardcoded path.

Why this shape

Command-template interpolation only substitutes declared-field {placeholders} (and
runs them through sanitize_input), and the plugin validator rejects {placeholders}
that don't map to a field — so a runtime-computed path can't be injected cleanly through
a field. A reserved literal resolved centrally in the executor (which owns the run
lifecycle and can guarantee cleanup) is the minimal, robust fit, and matches the issue's
"per-task sandbox dir" suggestion.

amass's parser only consumes stdout, so the -dir graph store is pure scratch and safe
to remove after the run. In Docker mode the container is --rm; the host scratch dir is
still created unpredictably and cleaned up.

Tests

• Updated the three amass command-render tests that asserted /tmp/amass (they encoded
  the vulnerable path) and added a regression assert that it's gone and the sentinel is
  present.
• New testing/backend/unit/test_executor_scratch_dir.py: no-op without the token; the
  token resolves to a real existing directory; distinct/unpredictable across calls; all
  occurrences replaced; directory is 0700-private on POSIX.

pytest test_amass_plugin.py unit/test_executor_scratch_dir.py → 27 passed, 1 skipped.
Executor suite (48), plugin integrity/checksum (20) pass. validate_plugins.py → all 59
valid. ruff clean.

Notes

• No default guard weakened; this only removes a predictable-path weakness and adds
  cleanup. PLUGINS.md catalogue row for amass is unchanged.

@utksh1 please review this pr

[utksh1]

Thanks for adding the Docker bind mount after the prior review. Please rebase and fix the failing backend-unit check, then add/confirm a regression test that the Docker command mounts the scratch directory at the exact path passed to the tool and that cleanup still occurs on a failed command.

[utksh1]

Thanks for adding the Docker bind mount after the prior review. Please rebase and fix the failing backend-unit check, then add/confirm a regression test that the Docker command mounts the scratch directory at the exact path passed to the tool and that cleanup still occurs on a failed command.

[utksh1]

The Docker bind-mount follow-up addresses the original concern, but the branch is still behind main and fails backend-unit. Please rebase, fix CI, and add an execution-level test proving the container receives the mounted path with cleanup after both success and failure.