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]

The host-side private scratch directory is not available inside the Docker sandbox: when docker_enabled is true, the command receives a host path but docker run does not mount it. Amass will fail to write there. Please mount the directory into the container with safe permissions (or create it inside the container), and add a regression test for the Docker command path as well as cleanup.