fix(store): report non-object inputSchema and surface schema findings

[syf2211]

Summary

Detect and report tool inputSchema values that are not valid object schemas (per MCP spec), record non-default JSON Schema dialects as observations, and surface all schema findings through check, export, and an opt-in --fail-on schema signal.

Motivation

Fixes #199 — servers like @modelcontextprotocol/server-filesystem can advertise tools whose inputSchema is only {"":"..."} with no type: object. SDKs reject these silently, but mcpsnoop check previously passed with zero warnings. Schema composition findings (oneOf, etc.) were computed but only shown in the TUI.

Changes

• Add nonObjectRoot finding for absent/invalid inputSchema roots; warn on the tools/list response frame
• Add nonDefaultDialect observation for non-2020-12 $schema on inputSchema and outputSchema
• Merge outputSchema dialect findings into per-tool Findings
• Print schema findings: section in check text output; add schema to --fail-on
• Include per-tool findings in JSON/text/HTML export
• Rank nonObjectRoot first in TUI schema headline order

Tests

go test ./internal/store ./cmd/mcpsnoop ./internal/exporter

All pass. Full suite except two pre-existing TUI style tests on upstream main (TestStreamRowShowsSupersededStatusInWarnStyle, TestStatusStyleWarnsOnTruncated).

Notes

• nonObjectRoot fails default check via the existing warn signal (warning on frame)
• Dialect/composition findings require --fail-on schema to fail CI
• Reviewed with composer-2.5 subagent before submission

[kerlenton]

Thank you!