feat(task): manual conversation checkpoint storage + Task trigger methods

[easonLiangWorldedtech]

feat(task): manual conversation checkpoint storage + Task trigger methods

Part of the Task Tree unified subtask management plan (issue #9, PR-5). Adds a conversation checkpoint — distinct from the existing git-based file checkpoints (RepoPerTaskCheckpointService) which snapshot the working tree. A conversation checkpoint snapshots the task's full message history so the user can restore the conversation to this point later.

New module: src/core/checkpoints/conversation-checkpoint.ts

Pure storage layer (no vscode/Task imports — fully unit-testable):

• saveConversationCheckpoint({taskDir, taskId, messages, summary?, now?}) → writes <taskDir>/checkpoints/<id>.json. The id is the creation timestamp; same-millisecond collisions get a -1, -2, ... suffix so no data is lost. Messages are deep-cloned (caller array never mutated); summary omitted from JSON when absent.
• listConversationCheckpoints(taskDir) → newest first (createdAt desc, then id desc on ties). Missing dir → empty list; corrupt files skipped rather than failing the listing.
• loadConversationCheckpoint(taskDir, id) → single checkpoint or undefined when missing.

Task trigger methods (manual only — no auto triggers in scope)

• Task.createConversationCheckpoint(summary?) — resolves the standard task dir via getTaskDirectoryPath(this.globalStoragePath, this.taskId) and snapshots structuredClone(this.clineMessages), returning the saved checkpoint.
• Task.listConversationCheckpoints() — lists this task's checkpoints newest first.

Tests (~290 lines per plan)

• src/core/checkpoints/__tests__/conversation-checkpoint.spec.ts (new, 8 cases): timestamp-named file + returned shape; summary omitted/included correctly; same-millisecond collision suffixing with both files coexisting; deep-clone isolation; empty-list on missing dir; newest-first ordering; corrupt-file tolerance; load-by-id + undefined when missing.
• src/__tests__/conversation-checkpoint-task.spec.ts (new, 2 cases): integration through the real Task.prototype methods bound onto a minimal double — verifies the standard <storage>/tasks/<taskId>/checkpoints/ layout and newest-first listing. No as any; single documented as unknown as Task cast on the stub.

Verification

• pnpm run check-types — 11/11 packages pass
• New suites: conversation-checkpoint unit 8/8; Task integration 2/2
• Regression: Task.spec.ts 96/96 unchanged (Task.ts touched)
• Full turbo lint (--max-warnings=0) clean; zero eslint suppression count increases