feat: HTTP/SSE transport for agentflare's own MCP server

[getappz]

Summary

Adds an opt-in Streamable-HTTP transport alongside the existing stdio transport for agentflare's top-level MCP server, so one persistent agentflare process can serve multiple concurrent agent sessions instead of spawning a full process per session.

• Arc-shared AgentflareMcp across sessions
• Loopback-only default (no auth yet — documented limitation, not a silent gap)
• agentflare mcp --http opt-in flag

Context

Investigated in item #58: agentflare's downstream gateway calls already reuse persistent connections (ensure_connected()), but the real bottleneck was one level up — the top-level MCP server itself only ran over stdio, so every connecting agent session cold-started its own full process with its own registries/DB connections.

Known follow-ups (not blocking, tracked separately)

• Per-process agent identity conflates multi-agent handoff over the shared HTTP transport
• No dedicated concurrent-request test for run_http yet

Test plan

• Rebased cleanly onto current master (was 74 commits behind)
• cargo build --bin agentflare — clean
• cargo test --bin agentflare mcp_server — 250 passed, 0 failed, 2 ignored (manual benchmarks)

Closes item #58.

[getappz]

Tracking this PR for automated review (flared:c997d745ae66).

[coderabbitai]

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Reviewing files that changed from the base of the PR and between 221e512 and 498ce7a.

📒 Files selected for processing (3)

• Cargo.toml
• src/cli/mcp.rs
• src/mcp_server.rs

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