subagents: add effort param, default to medium, document in skill

claude-plugins/plugins/base/skills/claude-subagents/SKILL.md

@@ -24,8 +24,8 @@ Your container's `subagent` MCP server (`hive-subagent-daemon`) runs this
 skill's recipe for you:
 
 ```
-start(name, prompt_file, model?, trigger?)
-continue(name, prompt, model?)
+start(name, prompt_file, model?, effort?, trigger?)
+continue(name, prompt, model?, effort?)
 status(name)
 interrupt(name, force?)
 ```
@@ -41,6 +41,37 @@ Model choice, prompt hygiene, splitting big batches, verify-then-report —
 everything else in this skill — applies exactly the same whether you're
 calling the tool or thinking through the recipe by hand.
 
+## Effort
+
+An omitted `effort` defaults to `medium` here — cheaper than claude's own
+model default (`high` on most models), a deliberate cost-conscious choice
+for subagent work specifically, same spirit as "cheaper-than-you" model
+choice above. Raise it explicitly when the task is complex enough to
+actually need deeper reasoning, not as a reflex.
+
+Anthropic's own levels (per current Claude Code docs — names/availability
+are model-dependent, check before relying on an exact list): `low`,
+`medium`, `high`, `xhigh`, `max`. Each trades token spend for capability:
+
+- **`low`** — short, scoped, latency-sensitive tasks that aren't
+  intelligence-sensitive.
+- **`medium`** — cost-sensitive work that can trade off some intelligence.
+  This skill's default for subagent work.
+- **`high`** — balances token usage and intelligence; Anthropic's own
+  recommended default for most _interactive_ coding tasks (not what this
+  skill defaults subagents to — see above).
+- **`xhigh`** — deeper reasoning at higher token spend.
+- **`max`** — demanding tasks only; diminishing returns and overthinking
+  are a real risk, per Anthropic's own guidance — don't reach for it as a
+  default.
+
+Anthropic's guidance: treat effort as a general preference, not a
+task-by-task dial — raise it if a subagent keeps skipping files, not
+running tests, or not double-checking its own work; lower it for routine
+work where quality hasn't suffered. Changing effort between turns on the
+_same_ session invalidates prompt caching, so pick a level for the whole
+session rather than flipping it turn-to-turn.
+
 ## Prompt hygiene - this is where batches succeed or fail
 
 - **Concrete constants, not "figure it out":** exact ids, field names,