docs(prd): refine PRD + add SRE-centric React UI overhaul with permalinks

[ethompsy]

What

A docs-only change to docs/reqs/main.md: a multi-agent refinement of the existing PRD plus a new forward-looking workstream. No code changes.

1. Refinement of the existing (delivered) requirements

Folds decisions discovered during implementation back into the PRD so it's falsifiable and self-consistent. All resolved from existing context (architecture spec, UX spec, completed plan) — no scope changes to delivered work.

• Detection: FR5 rate-vs-cumulative detection + infra denylist + 4σ floor; FR8 persisted cooldown (30m Completed / 60m Failed); FR9 ewma_warmup_samples semantics.
• Lifecycle: FR11 concurrency cap (defer-not-drop); FR13 orphaned-Running reconcile; NFR12 no false-positive burst on restart.
• UI: FR22/23/26/27 empty/failed/SSE states; FR41–44 sidebar state machine; FR32 auto-refresh; new NFR18 (WCAG 2.1 AA, focus-visible, prefers-reduced-motion).
• Acceptance: two-clocks timing note, an "evidence-backed" pass condition, and a 3/3 run protocol; new Open Questions Q1–Q3 (deferred, non-blocking).

2. New section — SRE-Centric React UI Overhaul (Phase 3 / v0.2.0)

A forward-looking workstream, sequenced after the Phase 1 DoD.

• Incremental Flask-BFF migration, incident-triage hero first (not a big-bang SPA).
• "First-seconds" information model (which service / component / problem state / severity / since-when), with a data-source column.
• FR45–53, NFR19–24, plus first-increment + workstream DoDs and post-rollout success metrics.
• FR48 blast radius explicitly gated on RFC 0001 Phase 3 (a later increment — the correlation API doesn't exist yet).
• FR51 component library built on the existing dark-first Tailwind tokens → the input for a Claude Design /design-sync pass (this is the React layer whose absence currently blocks design-sync).
• FR53 permalinks: every migrated view is a shareable URL with per-surface state (list filter / detail id + #step), deep-linked via the BFF serving the React shell; ephemeral chrome (sidebar, scroll) stays local.
• Open Questions R1–R6 (R2 resolved: Flask BFF).

Why

• Make the delivered PRD precise enough to be falsifiable and to onboard future contributors.
• Capture the next UI direction (SRE-centric, React, permalinkable) as reviewed requirements, and unblock a future Claude Design pass.

Review process

Each part went through a refine-requirements-style multi-agent review (Product Manager / Tech Lead / Designer) with a second verification cycle; findings were resolved before commit (e.g., FR48/RFC-0001 dependency, BFF routing precedence, permalink per-surface state, design-token continuity).

Notes for the reviewer

• Docs only — no behavior change; the completed plan docs/plans/main.md is intentionally untouched.
• Open Questions R1/R3–R6 and Q1–Q3 are the natural inputs to a future implementation-plan run (to a new plan path).