Flatten Git service layer and switch server paths to base dir

[juliusmarminge]

Summary

• Flattened the git service abstraction into GitCore, removing the separate GitService layer and updating downstream consumers and tests.
• Introduced base-directory driven path derivation so state, logs, worktrees, and attachments are resolved from T3CODE_HOME / --base-dir.
• Updated desktop and server bootstrapping, docs, and integration tests to use the new derived path layout.
• Adjusted attachment resolution to work from an explicit attachments directory instead of the full state directory.

Testing

• Not run (PR content only).
• Existing test coverage was updated across git, orchestration, attachment, and server config layers.
• Project checks expected before merge: bun fmt, bun lint, bun typecheck, and bun run test.

Note

Flatten Git service layer by merging GitService execution into GitCore

• Removes GitService as a dependency layer; GitCore now owns git process execution directly via an internal execute(input) method built with ChildProcessSpawner
• Default execution applies a 30s timeout and 1MB output cap per stream; both are configurable per-call via ExecuteGitInput
• CheckpointStoreLive no longer auto-provides a Git implementation — callers must explicitly wire a GitCore layer when composing the environment
• Server wiring in serverLayers.ts, test harnesses, and integration fixtures are updated to provide GitCoreLive directly
• Risk: CheckpointStoreLive is a breaking change for any caller that previously relied on it bundling GitServiceLive internally

^{Macroscope summarized d100711.}

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