feat(merge): unify planning conflicts onto the resolver + 3-pane VM foundation

Route planning unit-merge conflicts through ConflictResolverViewModel
(OpenForPlanningAsync) and delete the old ConflictResolutionViewModel dialog.
Add active-file 3-pane reconstruction (MergeFile OursText/TheirsText/ResultText,
ActiveFile, SelectFileCommand, active-file readout) as the VM foundation for the
Rider-style editor. Seam preserved; Ui.Tests 128/128.

src/ClaudeDo.Ui/CLAUDE.md

@@ -22,7 +22,7 @@ ViewModels/
   Modals/     — About, Diff, ListSettings, Merge, RepoImport, Settings (+ Settings/ tab VMs),
                 UnfinishedPlanning, WeeklyReport, WorkerConnection, Worktree,
                 WorktreesOverview, UnifiedDiffParser
-  Planning/   — PlanningDiffViewModel, ConflictResolutionViewModel
+  Planning/   — PlanningDiffViewModel
   Conflicts/  — ConflictResolverViewModel + ConflictModels (MergeFile/MergeFileSegment/MergeConflictBlock)
 Views/        — mirrors the VM layout; Islands/Detail/ holds TaskHeaderBar,
                 DescriptionStepsCard, WorkConsole; plus AgentStripView, SessionTerminalView
@@ -41,7 +41,7 @@ Design/     — Tokens.axaml (design tokens; merged before styles) + IslandStyle
 - **NotesEditorViewModel** — day navigator + bullet CRUD for daily notes via `INotesApi`.
 - **Modal VMs** — `SettingsModalViewModel` (four tabs: General, Worktrees, Files prompt-paths, Prime Claude incl. `DailyPrepMaxTasks` + prime-schedule rows), `ListSettingsModalViewModel` (name, working dir, commit type, per-list Model/SystemPrompt/AgentPath/MaxTurns with inherited-badge + reset, delete list), `RepoImportModalViewModel` (bulk-create lists from git repos found under chosen parents; already-wired repos disabled), `WeeklyReportModalViewModel` (range pickers default "since last standup weekday → today", cached per range, markdown via MarkdownView), `MergeModalViewModel` (single-task merge form, called from the diff modal), `WorktreesOverviewModalViewModel` (global/per-list worktree rows, batch merge + state ops), `UnfinishedPlanningModalViewModel` (Resume/FinalizeNow/Discard for a draft planning session), `WorkerConnectionModalViewModel` (offline help), `AboutModalViewModel`.
 - **Diff stack** — `UnifiedDiffParser` (static; parses `git diff` output into `DiffFileViewModel`s, detecting added/deleted/renamed/binary files and per-line numbers; `Flatten` injects file-header rows for a combined single-pane view). `DiffModalViewModel` has two modes: live worktree (branch diff vs base, with a Merge action) and commit-range `base..head` (`FromCommitRange = true` — shows a merged task's diff after its worktree is gone; no merge action). The view renders a file list with binary/empty placeholders via `DiffLinesView`.
-- **Planning/Conflicts** — `PlanningDiffViewModel` (per-subtask diffs via `GetPlanningAggregateAsync`, toggle to combined integration-branch diff, conflict warnings), `ConflictResolutionViewModel` (unit-merge conflict dialog: conflicted files, open in VS Code, continue/abort planning merge), `ConflictResolverViewModel` (in-app **3-way merge editor** for single-task merges: starts the conflict merge, parses each conflicted file into stable/conflict `MergeFileSegment`s via the worker's `GetMergeConflictDocuments`, flattens conflicts for one-at-a-time navigation, per-block Accept Ours/Base/Theirs/Both + editable result, composes each file and writes via `WriteConflictResolution`, continue/abort). The view (`Views/Conflicts/ConflictResolverView`) renders a 3-column Base|Ours|Theirs + result using **AvaloniaEdit** with TextMate syntax highlighting by file extension (theme `StyleInclude` in `App.axaml`); editors are synced in code-behind since AvaloniaEdit's `Text` isn't cleanly bindable.
+- **Planning/Conflicts** — `PlanningDiffViewModel` (per-subtask diffs via `GetPlanningAggregateAsync`, toggle to combined integration-branch diff, conflict warnings), `ConflictResolverViewModel` (in-app **3-way merge editor** for both single-task and planning unit-merge conflicts: single-task starts the conflict merge, parses each conflicted file into stable/conflict `MergeFileSegment`s via the worker's `GetMergeConflictDocuments`, flattens conflicts for one-at-a-time navigation, per-block Accept Ours/Base/Theirs/Both + editable result, composes each file and writes via `WriteConflictResolution`, continue/abort; **planning mode** via `OpenForPlanningAsync(parentId, subtaskId)` loads the current subtask's mid-merge conflicts without re-starting the merge and routes continue/abort to `ContinuePlanningMerge`/`AbortPlanningMerge`, so a unit-merge conflict re-opens the editor per subtask via the `PlanningMergeConflict` broadcast). The view (`Views/Conflicts/ConflictResolverView`) renders a 3-column Base|Ours|Theirs + result using **AvaloniaEdit** with TextMate syntax highlighting by file extension (theme `StyleInclude` in `App.axaml`); editors are synced in code-behind since AvaloniaEdit's `Text` isn't cleanly bindable.
 
 ## Services