feat: writes via chat — Reginald proposes, you approve inline (P4)

Completes "chat is the write-path" (decisions.md D1). Ask Reginald to re-estimate
or reprioritize an issue; it formulates a proposal, you approve it inline, and the
write runs through the same guarded apply_changes engine the Issue screen uses.
The model never writes — it proposes; the app owns approval + execution.

core (@commitea/core):
- propose_change tool declaration + REGINALD_SYSTEM updated ("never claim a change
  is applied; you propose, the human approves").
- proposalsFor(args, currentLabels, title): pure — builds the concrete, non-noop
  ChangeProposal(s) (change + label diff) for a propose_change request, dropping
  invalid/unchanged axes. ChangeProposal / ProposeChangeArgs types.

app:
- model bridge executes propose_change by planning against the issue's current
  labels (no write) and returns the proposals with the turn.
- useChat surfaces pending proposals + approve/dismiss; approve calls onApplyChange
  (AppShell's guarded handler → PUT + board/forecast refetch), dismiss leaves it.
- ChatPanel renders each proposal as a propose-approve card (diff + Approve/Dismiss).

Verified: 101 core tests green (4 proposalsFor added), desktop typecheck clean,
14 fixture e2e green. Gated live e2e against gemma-4-26b: "Set the estimate on #3
to est/5d" → Reginald proposes "est/2d → est/5d" as an inline card, says it's
*proposed* not done; Dismiss leaves the repo untouched. The approve→write path is
the #41 engine (separately verified change→revert).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/core/src/changes/apply-changes-v0.ts

@@ -57,3 +57,48 @@ export function describeChange(plan: LabelPlan): string {
   const to = plan.added.length ? plan.added.join(', ') : '∅'
   return `${from} → ${to}`
 }
+
+/** A change the agent proposes: the concrete op + its diff, ready for approve-then-apply. */
+export interface ChangeProposal {
+  change: IssueChange
+  plan: LabelPlan
+  issueTitle?: string
+}
+
+/** What the `propose_change` tool accepts — a target issue and the axes to set. */
+export interface ProposeChangeArgs {
+  issue: number
+  estimate?: EstimateLabel
+  priority?: PriorityLabel
+}
+
+function isEstimate(v: unknown): v is EstimateLabel {
+  return typeof v === 'string' && (ESTIMATE_LABELS as readonly string[]).includes(v)
+}
+function isPriority(v: unknown): v is PriorityLabel {
+  return typeof v === 'string' && (PRIORITY_LABELS as readonly string[]).includes(v)
+}
+
+/**
+ * Build the concrete, non-noop proposals for a `propose_change` request against
+ * an issue's current labels. Invalid or unchanged axes are dropped — the agent
+ * proposes only real changes, and never a label outside the est/* · p/* axes.
+ */
+export function proposalsFor(
+  args: ProposeChangeArgs,
+  currentLabels: string[],
+  issueTitle?: string,
+): ChangeProposal[] {
+  const out: ChangeProposal[] = []
+  if (isEstimate(args.estimate)) {
+    const change: IssueChange = { kind: 'reestimate', issue: args.issue, estimate: args.estimate }
+    const plan = planIssueChange(currentLabels, change)
+    if (!plan.noop) out.push({ change, plan, issueTitle })
+  }
+  if (isPriority(args.priority)) {
+    const change: IssueChange = { kind: 'reprioritize', issue: args.issue, priority: args.priority }
+    const plan = planIssueChange(currentLabels, change)
+    if (!plan.noop) out.push({ change, plan, issueTitle })
+  }
+  return out
+}