feat(alerts): full-page escalation chain editor - #2436

🟢 Confidence 8/10 · likely safe to merge. Held at 8 because 2 reviewed files were not read.
_{no findings · tests covered · risk medium · 2 files not read · 1 fix not yet re-checked against the whole diff}

Moves escalation chains from a dialog to a full page (/alerts/escalation-chains/new and /:chainId) with inline target chips, per-step menus, a delete confirm shared with the hub and an unsaved-changes guard. Both open findings are fixed and nothing new blocks the merge. The open McpQueryError issues come from the MCP query registry in apps/ai (registry.js, withTableListOnUnknownTable), not the page registry this diff touches, so they are neither fixed nor worsened here; the deleted dialog's view went only partly read.

• AlertsEscalationChain/...New routes replace the chain dialog; hub links and the rule editor's "Create a chain" now navigate to the page
• chainDeleteConfirm lists the rules that start a chain and holds Delete while any remains
• pickChips and chainConnector add inline chips and the step-to-step "+" to the page kit
• isBlockingNavigation/blockedNavigation guard unsaved edits; viewers without alerts:manage get read-only

Fixed since the last review

• ✅ F4 · Delete confirm says "These rules" for a chain one rule starts
• ✅ F5 · Notes field has no test: ChangedDescription is never driven

Production impact

_{0c18b56 · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}