feat(mcp): add --json mode to pipeboard mcp

[nictuku]

Summary

• Adds --json persistent flag on pipeboard mcp (applies to both tools-list and tools-call).
• Emits a clean structured envelope on stdout, suppresses status chatter on stderr, and silences cobra usage so parsers do not have to scan for the first {.
• Auto-unwraps the MCP content[].text wrapper and parses inner JSON when possible, so callers no longer need to double-parse.

Shape

• Success: {"ok": true, "data": <parsed>}
• Tool-level error (isError: true): {"ok": false, "error": "...", "data": "..."} + exit 1
• Transport / auth / network error: {"ok": false, "error": "..."} + exit 1
• tools-list --json: {"ok": true, "total": N, "tools": [...]}

Motivation

Pipes like this used to need manual text munging and two-step parsing:

pipeboard mcp --server meta-ads-mcp tools-call --tool get_creative_details --args '{...}' 2>&1 > out
python -c "idx=raw.find('{'); d=json.loads(raw[idx:]); inner=json.loads(d['content'][0]['text']) ..."

Now:

pipeboard mcp --json --server meta-ads-mcp tools-call --tool get_creative_details --args '{...}' | jq '.data.object_story_spec.page_id'

Test plan

• go build ./...
• go vet ./...
• go test ./cmd/... (4 new tests cover unwrap, isError, passthrough, error envelope)
• Smoke test with a bogus token returns {"ok": false, "error": "..."} and exit 1