fix: clarify category gate failure message for inclusive threshold

[laishettikarthik-tech]

Closes #209

What this PR does

• Fixes confusing failure message in category_gate_failures that
  showed "Passed" while exiting with code 1
• Message now explicitly shows score >= limit with "(inclusive gate)"
• Updated --fail-on-category help text to explain inclusive semantics
• Added 3 unit tests for boundary conditions in test_category_gates.py

Changes

• src/mcts/report/data.py — clearer failure message with score/limit
• src/mcts/cli/main.py — updated --fail-on-category help text
• tests/test_category_gates.py — 3 new boundary tests

Before

"Excessive Permissions scored Passed (limit 0)" → exit 1

After

"Excessive Permissions: risk score 0 >= limit 0
(inclusive gate — 'Passed' is category label, not CI result)" → exit 1

[laishettikarthik-tech]

Note for maintainer: This PR contains fixes for two separate issues:

1. Closes fix: clarify category gate failure message for inclusive threshold #231 — clarify category gate failure message (data.py + main.py + tests)
2. Closes [BUG] Deduplicate duplicate Docker FROM findings in supply-chain scan #190 — deduplicate Docker FROM findings (supply_chain.py + tests)

Both were committed to the same branch. Happy to split into separate PRs
if preferred — just let me know!

[hello-args]

@laishettikarthik-tech its fine for now, but please make sure to split into different branches when issues are unrelated.
Happy to merge if checks are good!!

[laishettikarthik-tech]

Thanks for the feedback. Apologies for the failing checks. I'm looking into the CI failures and will submit a fix shortly so the tests pass cleanly.