core: add handoff prompt template for extracting session context

packages/opencode/src/session/prompt/handoff.txt

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+Extract relevant context from the conversation above for continuing this work. Write from my perspective (first person: "I did...", "I told you...").
+
+Consider what would be useful to know based on my request below. Questions that might be relevant:
+
+- What did I just do or implement?
+- What instructions did I already give you which are still relevant (e.g. follow patterns in the codebase)?
+- What files did I already tell you that's important or that I am working on (and should continue working on)?
+- Did I provide a plan or spec that should be included?
+- What did I already tell you that's important (certain libraries, patterns, constraints, preferences)?
+- What important technical details did I discover (APIs, methods, patterns)?
+- What caveats, limitations, or open questions did I find?
+
+Extract what matters for the specific request below. Don't answer questions that aren't relevant. Pick an appropriate length based on the complexity of the request.
+
+Focus on capabilities and behavior, not file-by-file changes. Avoid excessive implementation details (variable names, storage keys, constants) unless critical.
+
+Format: Plain text with bullets. No markdown headers, no bold/italic, no code fences. Use workspace-relative paths for files.