feat(pi): add tldr prompt command

.pi/prompts/tldr.md

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+---
+description: Summarize an agent response
+argument-hint: "[source request]"
+---
+
+Summarize `${ARGUMENTS:-the preceding agent response}`.
+Keep the summary compact.
+Use no fixed format.

docs/00 INBOX/ai/tldr-design-tree.md

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+# TL;DR command design tree
+
+This tree reviews the implementation decisions in `tldr-design.md`.
+
+## Settled decisions
+
+- **Command form and scope:** Add the project prompt template `.pi/prompts/tldr.md`.
+- **Explicit source interface:** Interpret command arguments as a free-form source request.
+- **Default source:** Summarize the preceding agent response when no source request exists.
+- **Summary shape:** Produce a compact summary without a fixed format.
+- **Unavailable or unclear sources:** Add no special handling policy.
+
+## Decision tree
+
+- Command behavior
+  - Source selection
+    - Settled: Use the preceding agent response without arguments.
+    - Settled: Interpret arguments as a free-form source request.
+  - Output
+    - Settled: Produce a compact summary without a fixed format.
+    - Summary language
+      - Settled: Do not prescribe a language policy.
+  - Error behavior
+    - Settled: Add no special handling policy for unavailable or unclear sources.
+
+## Frontier
+
+The frontier is empty.
+
+## Shared understanding pending confirmation
+
+The project gets a `/tldr` prompt command in `.pi/prompts/tldr.md`.
+Without arguments, it summarizes the preceding agent response.
+With arguments, it interprets them as a free-form source request.
+It produces a compact summary with no fixed format or language policy.
+The prompt adds no special policy for unavailable or unclear sources.