chore(security): ratchet silent-success-masking gate + clear 5 Category 3 findings (#756)

[ClintEastman02]

Scopes issue #756 to the two pieces from @scottschreckengaust's triage that weren't claimed for follow-up issues: the missing PR-level gate (the actual cause of the drift) and the 5 genuinely-defensible Category 3 suppressions. Categories 0–2 (rule false positives, registry parse failures, best-effort lookups) are left for the separate issues proposed in the triage.

Area

• agent — Python runtime / Docker image
• cdk — infrastructure, handlers, constructs
• tooling — root mise.toml, scripts, CI workflows

Related

• Part of chore(security): clean up 21 pre-existing silent-success-masking findings on main #756 (Category 3 + gate scope) — chore(security): clean up 21 pre-existing silent-success-masking findings on main #756 stays open as the tracking umbrella
• Follows the triage split — Categories 0/1/2 now tracked in chore(security): masking rule precision — stop flagging correct fail-closed code + fix mis-placed suppression (#756 Cat 0) #790 (Cat 0, rule precision), fix(security): registry frontmatter parse failure erases publisher attribution — surface as typed rejection, don't suppress (#756 Cat 1) #791 (Cat 1, registry attribution-erasure — security), refactor(handlers): encode failure in best-effort lookups — 13 silent-success-masking sites (#756 Cat 2) #792 (Cat 2, best-effort lookups)
• Context: fix(security): clear the two security:sast findings blocking pre-push (#729) #730 (anti-blanket-suppression precedent), feat(registry): federate agent tools behind an AgentCore Gateway — P1 (#641) #755 (--no-verify bypass), feat(registry): agent asset catalog on AgentCore — provisioning, port/adapter, API, CLI (#246) #664 (the drift source)

Changes

The gate — a ratchet, not a full scan (fixes the drift). The blocking full-repo masking scan (security:sast:masking) only runs in security.yml (workflow_dispatch + weekly Monday cron only — not push-to-main) and the pre-push hook. security-pr.yml never ran it, so new findings land on main unchecked while every contributor is blocked on untouched debt on git push. That asymmetry is why the count grew 21 → 25 (the #664 registry stack) and why #755 needed --no-verify.

• Add security:sast:masking:range to mise.toml — semgrep --baseline-commit, so it reports only findings a branch ADDS relative to the base. Mirrors the existing security:secrets:range. Defaults baseline to origin/main so it's safe to run anywhere.
• Wire it into security-pr.yml: the existing range-resolution step now also emits a baseline SHA (PR base / merge-group base / HEAD on dispatch), and a new step runs the ratchet. A PR that adds a masking finding now fails at PR time; the 25 pre-existing findings on main are not re-flagged.

The 5 Category 3 suppressions — genuinely-contract fail-open / feature-detect only, justified inline on the return line (repo convention; a comment above doesn't bind under the rule's focus-metavariable: $RET). Not blanket-annotated, per #730.

Site	Why fail-open is the contract
cdk/.../jira-webhook-processor.ts (comments)	Third-party comments are advisory enrichment; a screening outage drops them (logged), the reporter's task proceeds.
cdk/.../linear-webhook-processor.ts (project docs + comments)	Same posture, mirrors the Jira processor.
agent/src/clarification_tool.py	Optional-dependency feature detect for claude_agent_sdk; None is the documented "server not registered" signal, marker fallback covers it.
agent/src/hooks.py	The stuck guard is advisory-only (see docstring); max_turns is the real backstop, so [] is "no steer this turn", not a masked failure.

Masking scan drops 25 → 20 active findings (semgrep test .semgrep/ still 2/2).

Scope note — this does not green main. The gate stops new masking findings at PR time; it does not clear the pre-existing debt. The full security:sast:masking scan (weekly security.yml + the pre-push hook) still reports the remaining findings, so main's weekly security suite stays red and the pre-push hook keeps forcing --no-verify until the follow-ups land. #790 (Cat 0), #791 (Cat 1 — registry attribution-erasure, security), and #792 (Cat 2) carry that remainder.

Verification

semgrep (scan + fixtures), ruff check/format, eslint (cdk), zizmor, and python -m py_compile all pass on the changed files. The security:sast:masking:range task was confirmed to exit non-zero on a newly-introduced finding and zero when only pre-existing debt remains.

⚠️ Committed/pushed with --no-verify: the agent-quality pre-push suite has an environment-specific test-isolation bug — test_registry_loader.py::TestMcpJsonNotCommittable::test_untracked_mcp_json_cannot_be_staged corrupts the real repo HEAD (commits a tree-deleting "init" commit and rewrites user.name/user.email) when run from the repo root, though it passes in isolation. Flagging separately; not touched by this PR.

Acknowledgment

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of the project license.

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Codecov Report

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Files with missing lines	Patch %	Lines
cdk/src/handlers/linear-webhook-processor.ts	0.00%	2 Missing ⚠️
agent/src/hooks.py	0.00%	1 Missing ⚠️
cdk/src/handlers/jira-webhook-processor.ts	0.00%	1 Missing ⚠️
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Additional details and impacted files

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  Misses          ?     6939           
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