feat(plugin): expose session messages

[rekram1-node]

Exposes session message reads to plugins.

• Adds ctx.message.list (MessageApi) to Effect and Promise plugin contexts.
• Adds ctx.session.message.get to SessionDomain (Effect + Promise).
• Implements both in PluginHost with server-compatible pagination (default limit 50, base64url cursors, cursor+order rejection).
• Adds MessageApi to promise client api types.
• Covers Effect and Promise paths in packages/core/test/plugin-message.test.ts.

Also repairs two latent v2 test breakages surfaced by affected-package runs (neither is caused by this PR):

• fix(codemode): expect fs.write in skipped operations (added by feat(server): add fs.write endpoint #49466, test never updated).
• fix(app): include step start time in timeline fixture (schema requires data.started since provider-latency change, fixture never updated).