Align apply_patch results with Codex and support SSH backends

[AMagicPear]

Summary

This PR brings the extension's apply_patch result handling closer to Codex while preserving the repository's existing src/ layout.

Changes

• Persist final file changes as compact Codex-style hunks-only unified diffs.
• Keep final details.changes available for TUI expansion/collapse, including Ctrl+O re-rendering.
• Keep streaming preview behavior separate from the compact final result.
• Add injectable filesystem operations so SSH sessions apply patches through the remote filesystem backend.
• Wire the SSH backend through the pi event bus to avoid a direct extension dependency.
• Add support for add, delete, update, move, fuzz, partial failure, and progress details.
• Add a root index.ts loader entry while preserving the main implementation under src/, so pi displays the real extension directory name instead of src.
• Keep standard GPT providers as the default and allow private provider IDs through the local ~/.pi/agent/pi-apply-patch.json configuration file without committing user-specific providers.

Issue coverage

Closes #23
Closes #16

Verification

• npm run check
• npm test (55/55)
• git diff --check
• Real pi loader smoke test with pi -e ./index.ts --no-session --mode json --offline
• Remote filesystem backend test

The local provider configuration is intentionally outside the repository and is not included in this PR.

[code-yeongyu]

Thanks for this PR. I have merged the non-SSH parts it overlaps with via separate PRs:

• Compact final diff preview (Final apply_patch result should keep diff preview after completion #23) — landed in Fix apply_patch path handling and final diff preview #22 (final result retains preview).
• Custom/private provider config — the custom Responses API provider activation landed in feat: support custom Responses providers #33 (gates on model.api + gpt- prefix instead of a provider allowlist).

I am excluding the SSH filesystem backend from this round: it introduces an injectable filesystem-operations layer wired through the pi event bus, which is a meaningful architectural addition I want to evaluate separately rather than merge bundled with the result-formatting changes.

If you would like the SSH backend to land, please split it into a standalone PR with just the injectable filesystem operations + event-bus wiring + SSH tests, and I will review it on its own merits. The root index.ts loader entry and bun.lock changes can come with it.