fix(#1626): make the security gate severity-aware via per-threat severity (#1635)

workflow.security_block_on was documented as the minimum threat severity
that blocks advancement, but threats carried no severity and the auditor's
threats_open count (the SECURITY.md gate field) counted every open threat
regardless of severity — so the threshold had no effect, and the auditor's
block_on vocabulary (open/unregistered/none) did not even match the config
enum (critical/high/medium/low/none).

- planner: add a Severity column to the STRIDE threat register; assign
  severity per threat.
- auditor: read severity; reconcile the <config> block_on domain to the
  severity enum; redefine threats_open as the count of OPEN threats whose
  severity is at or above block_on (none => 0). Below-threshold opens are
  reported as non-blocking and excluded from threats_open.
- SECURITY.md template + planning-config.md reconciled.

No gate-check site changed: threats_open == 0 stays the gate everywhere;
only its computation is now severity-filtered.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

gsd-core/references/planning-config.md

@@ -276,7 +276,7 @@ Set via `workflow.*` namespace in config.json (e.g., `"workflow": { "research":
 | `workflow._auto_chain_active` | boolean | `false` | `true`, `false` | Internal: tracks whether autonomous chaining is active |
 | `workflow.security_enforcement` | boolean | `true` | `true`, `false` | Enable threat-model-anchored security verification via `/gsd:secure-phase`. When `false`, security checks are skipped entirely |
 | `workflow.security_asvs_level` | number | `1` | `1`, `2`, `3` | OWASP ASVS verification level. Level 1 = opportunistic, Level 2 = standard, Level 3 = comprehensive. Scales both planner threat-disposition rigor (which threats must be mitigated vs. accepted) and auditor verification depth (grep-level → boundary-placement check → full data-flow trace). See `gsd-core/references/security-asvs-levels.md`. |
-| `workflow.security_block_on` | string | `"high"` | `"high"`, `"medium"`, `"low"` | Minimum severity that blocks phase advancement |
+| `workflow.security_block_on` | string | `"high"` | `"critical"`, `"high"`, `"medium"`, `"low"`, `"none"` | Minimum threat severity that blocks phase advancement. The auditor counts only open threats at or above this severity toward the blocking gate (SECURITY.md `threats_open`); `none` disables severity blocking. |
 | `workflow.post_planning_gaps` | boolean | `true` | `true`, `false` | Post-planning gap report (#2493). After plans are generated, scans REQUIREMENTS.md and CONTEXT.md `<decisions>` against all PLAN.md files and emits a unified `Source \| Item \| Status` table. Non-blocking. Set to `false` to skip Step 13e of plan-phase. _Alias:_ `post_planning_gaps` is the flat-key form used in `CONFIG_DEFAULTS`; `workflow.post_planning_gaps` is the canonical namespaced form. |
 
 ### Ship Fields