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>

docs/CONFIGURATION.md

@@ -830,7 +830,7 @@ These keys live under `workflow.*` — that is where the workflows and installer
 |---------|------|---------|-------------|
 | `workflow.security_enforcement` | boolean | `true` | Enable threat-model-anchored security verification via `/gsd-secure-phase`. When `false`, security checks are skipped entirely |
 | `workflow.security_asvs_level` | number (1-3) | `1` | OWASP ASVS verification level. Level 1 = opportunistic, Level 2 = standard, Level 3 = comprehensive |
-| `workflow.security_block_on` | string | `"high"` | Minimum severity that blocks phase advancement. Options: `"high"`, `"medium"`, `"low"` |
+| `workflow.security_block_on` | string | `"high"` | Minimum threat severity that blocks phase advancement. The auditor counts only open threats at or above this severity toward the blocking gate; `none` disables severity blocking. Options: `"critical"`, `"high"`, `"medium"`, `"low"`, `"none"` |
 
 ---