Refactor runAt requests and harden broker flows

[ejgallego]

Summary

This PR packages a set of refactors and hardening changes around the standalone runAt plugin and the local Beam broker.

Main changes:

• split the plugin implementation into request-oriented modules and shared support modules
• keep RunAt/Plugin.lean as a thin registration file
• add focused request-surface regressions for goals, direct imports, save readiness, and save artifacts
• remove the dead legacy broker handler stack
• harden broker startup so initialize waits for the actual initialize response
• replace several internal broker string-prefix failures with a structured internal failure envelope
• prune stale handle snapshots into cheap tombstones while preserving contentModified behavior
• isolate the stopgap dependency scanner and stale direct-dependency hint shaping in Beam/Broker/Deps.lean
• move reusable sync/save support into Beam/Broker/SyncSaveSupport.lean
• demote lean-deps from the recommended CLI/skill surface to lean-beam experimental deps

Testing

Ran:

• bash tests/test.sh
• bash tests/test-broker-fast.sh
• bash tests/test-broker-slow.sh
• .lake/build/bin/runAt-request-surface-test
• .lake/build/bin/runAt-handle-lifecycle-test
• .lake/build/bin/beam-daemon-startup-handshake-test

Notes

• lean-deps remains as a compatibility alias for now, but docs and the recommended workflow now point to lean-beam experimental deps.
• Local generated .beam/ and .tmp/ directories from test runs were left untracked and are not part of the PR.

[ejgallego]

Superseded by #9, which respins the internal refactor/hardening work on top of current main and leaves the stale lean-deps surface churn out of the branch.