fix(#3531): merge routing_tier_defaults over manifest tier defaults (#3539)

* test(#3531): failing-first suite for routing_tier_defaults manifest merge

* fix(#3531): merge routing_tier_defaults over manifest tier defaults

* docs(#3531): document routing_tier_defaults merge-over-built-ins semantics

* fix(#3531): correct test helper scope, update folded #443 expectations, guard merge keys

* test(#3531): pin tiers in effort-sync and surface-axis fixtures post-merge

* chore(#3531): backfill changeset pr number

* fix(#3531): correct rebase resolution — keep both 3531 and 3533 test blocks intact

* test(#3531): pin inherit/effort fixtures to the layer that reaches tiered agents

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Co-authored-by: sim <sim@local>

docs/CONFIGURATION.md

@@ -1384,7 +1384,10 @@ The model-catalog's `reasoning_effort` per-tier hint is a legacy field kept for
 **Precedence (highest → lowest):**
 1. Invocation override (e.g. `--effort` flag on `resolve-execution`)
 2. `effort.agent_overrides[<agent-id>]`
-3. `effort.routing_tier_defaults[<light|standard|heavy>]`
+3. `effort.routing_tier_defaults[<light|standard|heavy>]`, **merged per-tier over the
+   built-in tier defaults** (`light: low`, `standard: high`, `heavy: xhigh`) — a partial
+   block fills its gaps from the built-ins instead of discarding them, and an invalid
+   value falls back to that tier's built-in ([#3531](https://github.com/open-gsd/gsd-core/issues/3531))
 4. `effort.default`
 5. `"high"` (Anthropic Opus 4.8 universal default)