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/index.ts

@@ -23,8 +23,8 @@ export type {
   GiteaRequestInit,
 } from './gitea/types.js'
 
-export { describeChange, planIssueChange } from './changes/apply-changes-v0.js'
-export type { IssueChange, LabelPlan } from './changes/apply-changes-v0.js'
+export { describeChange, planIssueChange, proposalsFor } from './changes/apply-changes-v0.js'
+export type { ChangeProposal, IssueChange, LabelPlan, ProposeChangeArgs } from './changes/apply-changes-v0.js'
 
 export {
   inferColumnV0,
@@ -85,6 +85,6 @@ export { pickModel } from './agent/model-router.js'
 export type { ModelRouter, TaskKind } from './agent/model-router.js'
 export { runAgentTurn } from './agent/agent-loop.js'
 export type { AgentStep, AgentTurn, ToolExecutor } from './agent/agent-loop.js'
-export { QUERY_PROJECT_TOOL, REGINALD_SYSTEM, REGINALD_TOOLS } from './agent/agent-tools.js'
+export { PROPOSE_CHANGE_TOOL, QUERY_PROJECT_TOOL, REGINALD_SYSTEM, REGINALD_TOOLS } from './agent/agent-tools.js'
 export { buildProjectView } from './agent/query-project.js'
 export type { ProjectSnapshot, ProjectView, QueryFilters } from './agent/query-project.js'