fix(tools): put body inside StructuredContent so it reaches the consumer

[WiktorStarczewski]

Summary

Follow-up to #14, which turned out to be only half the fix.

PR #14 made AskPeerClaudeOutput / SendPeerMessageOutput empty on the theory that MCP consumers preferring StructuredContent would fall back to Content when StructuredContent was just {}. Empirical test against Claude Code showed they don't: the structured channel is treated as the source of truth whenever it's present, even if empty, and Content is silently dropped. Net effect: every successful ask_peer_claude returned a literal {} to the calling model — body invisible.

This PR puts the body inside StructuredContent (where the consumer actually looks) and keeps it in Content as a redundant fallback for any consumer that does the opposite. The body is now reachable through whichever channel the consumer prefers.

Wire-shape changes

AskPeerClaudeOutput / SendPeerMessageOutput:

 type AskPeerClaudeOutput struct {
+    Body          string `json:"body,omitempty"`
+    TurnCount     int    `json:"turnCount,omitempty"`
+    ToolCallCount int    `json:"toolCallCount,omitempty"`
+    StopReason    string `json:"stopReason,omitempty"`
+    ElapsedMs     int64  `json:"elapsedMs,omitempty"`
+    ErrorSummary  string `json:"errorSummary,omitempty"`
 }

ReadSessionOutput gains the same body field (only populated for format="full"; format="json" still omits it).

The transcriptHeaderLine / withTranscriptHeader helpers introduced by #14 are removed — no longer needed now that metadata rides on its own structured fields.

Tests

• Existing happy-path tests for ask_peer_claude / send_peer_message updated to decode the body from StructuredContent rather than the (now-removed) inline header.
• New regression test TestAskPeerClaude_BodyReachesStructuredOnlyConsumer decodes via the structured channel only and asserts the body is non-empty — the actual rendering rule on Claude Code-class clients.
• TestReadSession_ReturnsMarkdownAndMetadata extended to verify meta.Body carries the rendered markdown.
• TestReadToolResult_NoStructuredContent is not affected — that tool already chose to drop StructuredContent rather than carry a body in it; whether to fold it into the same pattern is a separate decision (its primary use is large-sidecar reads, where the duplication might matter for size).

Version bump

0.3.1-dev → 0.3.2-dev. Same diagnostic motivation as last time: lets a calling Claude tell, via get_peer_info, whether the connected peer has the actual fix.

Test plan

• go vet ./... clean
• go test ./... — all packages green (modulo the pre-existing tailscale-test PATH-stub flake)
• New regression test fails on origin/main (would have caught fix(tools): inline transcript metadata into ask/send_peer_message body #14's incompleteness)