Sallyport 0.8.15

Sallyport 0.8.15

Hotfix: the MCP stdio shim (sp mcp) blocked on stdin with
FileHandle.read(upToCount:), which waits for a full buffer or EOF. A
live MCP host writes one small initialize request into an open pipe
and waits for the reply, so the shim never answered and every host —
Claude Code, Claude Desktop, Codex CLI, OpenCode — timed out after 30s
("failed to connect"). 0.8.14 shipped this regression; the agent ->
sp -> vault path was broken with every MCP host. Switch to
availableData so replies flush as requests arrive.

Also hardens the macOS CI lane against a stalled Sparkle binary-artifact
download (fail-fast pre-resolve with retries; persistent SPM scratch
dirs), with no effect on the shipped app.