feat: add descriptive messages to cloud action responses

[amir20]

Summary

• UpdateContainer now returns (bool, error) — true if updated, false if already up-to-date
• Added message field to ActionResult proto with human-readable descriptions for LLM consumption (e.g. "Successfully restarted container abc123")
• Cloud tool callers no longer need to construct English from raw action strings

Test plan

• go build ./... passes
• go test ./internal/cloud/ ./internal/agent/ ./internal/web/ ./internal/support/... all pass
• Verify cloud action responses return proper messages end-to-end

🤖 Generated with Claude Code

[claude]

• Bug - untested path: tools_test.go mock always returns (false, nil), so only the "already up-to-date" message path is exercised. The "successfully updated" branch has no test coverage.

• Magic string coupling: agent/client.go detects update success via progress.Status == "done" — tightly coupled to the string literal in docker_service.go. A shared constant would prevent silent breakage.

• pastTense default fallback: string(action) + "ed" produces incorrect English for irregular verbs. Fine for now since all current actions are explicit, but fragile if new actions are added without updating the switch.

• internal/web/actions.go still discards the bool return — intentional, but means the HTTP update endpoint can't surface "already up-to-date" to the frontend. Worth a TODO or comment if that's a known gap.