refactor(semantics): make complexity tiers descriptive, not gatekeeping

Why: tier rules forbidding @PRE/@POST on C2 cause agents to either
remove useful documentation or generate audit noise. Both outcomes are
worse than having slightly richer metadata.

Changed:
- Tier table in all 4 agent prompts: removed 'Forbidden' column,
  replaced with 'descriptive signal, NOT gatekeeper'
- All tags (@PRE/@POST/@RATIONALE/@REJECTED) welcomed at any tier
- semantics-core: complexity table collapsed to descriptive
- qa-tester: removed tier-based reject rules from P1 and Phase 1

Hard rules preserved: CONTRACT-FIRST, ANCHOR SAFETY, FRACTAL LIMIT,
RESURRECTION BAN. Everything else is descriptive intent.

.opencode/agents/fullstack-coder.md

@@ -38,16 +38,16 @@ You are a long-horizon agent operating across TWO stacks (Python backend + Svelt
 **CONTRACT-FIRST:** Before `def` or `<script>`, write `#region id [C:N] [TYPE Type] [SEMANTICS tags]`.
 Every function, class, component, and module MUST open with `#region`.
 
-**COMPLEXITY TIERS:**
+**COMPLEXITY TIERS** (descriptive signal, NOT a tag gatekeeper):
 ```
-C1 = anchor pair only      (DTOs, constants, trivial wrappers)
-C2 = C1 + @BRIEF           (utility functions, pure computations)
-     @RELATION allowed at C2, forbidden: @PRE @POST @SIDE_EFFECT @DATA_CONTRACT
-C3 = C2 + @RELATION required (multi-step logic with dependencies)
-     forbidden: @PRE @POST @SIDE_EFFECT @DATA_CONTRACT
-C4 = C3 + @PRE @POST @SIDE_EFFECT required (stateful orchestration)
-     @RELATION also required at C4, forbidden: @DATA_CONTRACT
-C5 = C4 + @INVARIANT (critical infrastructure), @DATA_CONTRACT recommended
+C1 = simple DTO/constant       (anchor pair)
+C2 = pure utility function     (+ @BRIEF)
+C3 = multi-step with deps      (+ @RELATION)
+C4 = stateful, side effects    (+ @SIDE_EFFECT)
+C5 = critical infrastructure   (+ @INVARIANT)
+
+Add @PRE/@POST/@RATIONALE/@REJECTED anywhere they document intent.
+The tier describes what the function IS, not what it's forbidden to carry.
 ```
 
 **ANCHOR SAFETY:** Every `#region` MUST have a matching `#endregion` with EXACT same ID.