feat(mcp): support push notifications

[Streamweaver]

Summary

Adds MCP resource push-notification support across the type, runtime, and kernel layers. This lets OpenFang subscribe to MCP resources, receive server-side notifications, bridge them into kernel events, and replay subscriptions after MCP reconnects.

Refs #1096.

Changes

• Adds typed MCP push notification, subscription, and event contracts.
• Extends the MCP runtime to subscribe to resources, handle push events, rate-limit notifications, and emit tool-list-change events for targeted capability refreshes.
• Bridges MCP push events through the kernel trigger path so agents can react to resource changes.
• Adds an mcp_resource_subscribe tool and matching capability/config support.
• Replays MCP subscriptions after reconnect so long-lived agents do not silently lose resource watches.

This is currently one focused draft PR. I can split it into stacked PRs if maintainers would prefer type contracts, runtime handling, kernel bridging, and tool surface changes reviewed separately.

Testing

• cargo fmt --all -- --check passes locally
• cargo check --workspace passes locally with openfang-desktop included
• cargo clippy --workspace --all-targets -- -D warnings passes locally with openfang-desktop included
• cargo test --workspace passes locally with openfang-desktop included
• CI check/test/clippy/format matrix passes
• cargo audit currently fails on an existing workspace dependency advisory: lettre 0.11.21 / RUSTSEC-2026-0141; fix is to upgrade to lettre >= 0.11.22
• Live integration tested (if applicable)

Security

• No new unsafe code
• No secrets or API keys in diff
• User input validated at boundaries