Full e2e tests using real agent, basic tests

[Soph]

Adding E2E tests executing prompts against a real agent and executing all hooks through claude code and git.

Main purpose is to validate large amounts of works and also to be used when a new release is made to validate again that the core functionalities are working.

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[Copilot]

Pull request overview

Adds a new e2e test suite that runs the CLI against a real coding agent (Claude Code today) to validate core workflows (enable/hooks, checkpoints, rewind) in a more production-like setup, intended for release validation.

Changes:

• Add mise tasks to run the new e2e test package with different agent selections.
• Introduce an e2e_test harness (test repo setup, CLI binary build, agent runner abstraction).
• Add several E2E scenarios covering basic workflow, checkpoint metadata, agent commits, and rewind behavior.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
mise.toml	Adds tasks to run E2E tests with real agents.
cmd/entire/cli/e2e_test/setup_test.go	Builds a shared entire binary in TestMain and configures PATH for hooks.
cmd/entire/cli/e2e_test/testenv.go	Provides E2E repo/CLI helpers (git init/commit, enable, rewind listing, agent execution).
cmd/entire/cli/e2e_test/agent_runner.go	Implements agent runners (Claude Code + Gemini placeholder) used by E2E tests.
cmd/entire/cli/e2e_test/prompts.go	Defines deterministic prompts used by scenarios.
cmd/entire/cli/e2e_test/assertions.go	Adds helper assertions for files, rewind points, and agent results.
cmd/entire/cli/e2e_test/scenario_basic_workflow_test.go	Validates baseline workflow (agent change → commit → checkpoint/metadata branch).
cmd/entire/cli/e2e_test/scenario_checkpoint_test.go	Validates checkpoint metadata/trailer behavior and ID format.
cmd/entire/cli/e2e_test/scenario_agent_commit_test.go	Covers agent committing during its turn and multi-session behavior.
cmd/entire/cli/e2e_test/scenario_rewind_test.go	Exercises rewinding to checkpoints across edits/commits/multiple files.