Docs: document thinking levels

docs/thinking.md

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+# Thinking Levels (/think directives)
+
+## What it does
+- Inline directive in any inbound body: `/t <level>`, `/think:<level>`, or `/thinking <level>`.
+- Levels (aliases): `off | minimal | low | medium | high`
+  - minimal → “think”
+  - low → “think hard”
+  - medium → “think harder”
+  - high → “ultrathink” (max budget)
+  - `highest`, `max` map to `high`.
+
+## Resolution order
+1. Inline directive on the message (applies only to that message).
+2. Session override (set by sending a directive-only message).
+3. Global default (`inbound.reply.thinkingDefault` in config).
+4. Fallback: off.
+
+## Setting a session default
+- Send a message that is **only** the directive (whitespace allowed), e.g. `/think:medium` or `/t high`.
+- That sticks for the current session (per-sender by default); cleared by `/think:off` or session idle reset.
+
+## Application by agent
+- **Pi/Tau**: injects `--thinking <level>` (skipped for `off`).
+- **Claude & other text agents**: appends the cue word to the prompt text as above.
+
+## Heartbeats
+- Heartbeat probe body is `HEARTBEAT /think:high`, so it always asks for max thinking on the probe. Inline directive wins; session/global defaults are used only when no directive is present.