fix(seahorse): prevent tool-call format leakage into LLM summaries

[MrTreasure]

Related: same symptom triggered by seahorse summaries

I just hit this exact class of bug — tool-call format leaking into user messages — but triggered through a different path: seahorse's partsToReadableContent.

The seahorse variant

partsToReadableContent (in pkg/seahorse/store.go) serializes tool calls as [tool_use: name, args: {...}] for FTS5 indexing. This format was also used as a fallback in two summary-generation paths (formatMessagesForSummary and truncateSummary in short_compaction.go). When summaries containing [tool_use: exec, args: {...}] were fed back into the LLM context, the model learned to mimic this format in its own responses — outputting mock tool calls as visible text content.

The pipeline

partsToReadableContent -> "[tool_use: exec, args: {...}]"
     -> seahorse summary / truncateSummary
     -> LLM context (as summary text)
     -> LLM mimics format in its content output
     -> User sees raw [tool_use: exec, args: {...}] in chat

Fix

PR incoming at: https://github.com/sipeed/picoclaw/pulls (MrTreasure:fix/seahorse-tool-call-leak)

Three changes:

1. short_compaction.go: Skip messages with empty content in summary/truncation paths — tool calls aren't conversational content
2. store.go: Change FTS5 format from [tool_use: name, args: {...}] to emoji markers — the bracket-colon pattern is what LLMs latch onto
3. agent_utils.go: Strip residual [tool_use: ...] from outbound messages as a safety net

Same root cause as this issue: LLM context leaking internal serialization format → model mimics it → user sees raw tool-call text. Different trigger (seahorse vs Doubao provider), same outcome.