refactor: consolidate TestRepo into single src/testing module

[max-sixty]

Two TestRepo structs existed — a lightweight one in src/testing.rs (74 lines, for unit tests) and a full-featured one in tests/common/mod.rs (~1500 lines, for integration tests). This consolidates them into one struct in src/testing/ that serves both.

The key blocker was the integration TestRepo's _snapshot_guard: insta::internals::SettingsBindDropGuard field — insta is a dev-dependency and can't appear in pub code in src/. Replaced with Option<Box<dyn Any>> (type-erased). Snapshot guard setup moved to the repo() rstest fixture.

What moved to src/testing/:

• TestRepo struct with all ~80 methods
• BareRepoTest, TestRepoBase trait
• mock_commands module
• Helper functions (env isolation, fixture copying, CLI command builders, wait utilities)
• Constants (TEST_EPOCH, timing constants, etc.)

What stayed in tests/common/:

• rstest fixtures (repo(), repo_with_*, etc.)
• Snapshot settings functions (all insta::Settings code)
• PTY functions (portable_pty code)
• Re-exports: pub use worktrunk::testing::*

Constructor semantics:

• TestRepo::new() — lightweight git init + identity (backward-compatible with unit tests)
• TestRepo::standard() — fixture copy with remote + worktrees + mock gh (what integration new() was)
• TestRepo::with_initial_commit() — lightweight + one commit
• TestRepo::empty() — git init with no commits

This was written by Claude Code on behalf of maximilian

CI failed — TestRepo::new() missing local git identity

[worktrunk-bot]

CI fails: all commands::picker::summary::tests panic with "Author identity unknown" on t.repo.run_command(&["commit", ...]).

Root cause: The old src/testing.rs::TestRepo::new() set identity in the repo's local git config via repo.run_command(&["config", "user.name", "Test"]). The new TestRepo::new() writes identity to an external gitconfig file (write_test_gitconfig) and uses it via GIT_CONFIG_GLOBAL in configure_git_env. But Repository::run_command() doesn't set GIT_CONFIG_GLOBAL, so tests that commit via t.repo.run_command() fail.

Fix options:

1. Have TestRepo::new() also set local git config identity (like the old version did), so repo.run_command() works without env setup
2. Change the picker tests to use t.run_git() / t.git_command() instead of t.repo.run_command()

Option 1 is simplest and most backward-compatible — the tests are calling a reasonable API (t.repo.run_command) and shouldn't need to know about env isolation.