feat: headroom wrap opencode / unwrap opencode CLI (#1105)

## Summary

This PR implements transparent `headroom wrap opencode` support without
asking users to edit OpenCode provider URLs, choose an extra CLI flag,
or maintain a static provider list.

The wrapper now lives at the runtime transport boundary: OpenCode keeps
its user/provider config, while Headroom intercepts outbound provider
traffic in-process and routes it through the local Headroom proxy.

## What changed

### Transparent OpenCode wrapping

- `headroom wrap opencode` injects the `headroom-opencode` plugin
through `OPENCODE_CONFIG_CONTENT`.
- Existing OpenCode provider URLs are preserved. We do not rewrite user
config URLs to point at Headroom.
- Existing `OPENAI_BASE_URL` and `ANTHROPIC_BASE_URL` env vars are
preserved.
- Local OpenCode traffic, localhost traffic, and Headroom proxy traffic
bypass the shim to avoid loops.

### Runtime transport interception

- Added an OpenCode plugin transport shim that wraps:
  - `globalThis.fetch`
  - `http.request` / `http.get`
  - `https.request` / `https.get`
- External provider calls are routed to the local Headroom proxy.
- The original upstream origin is passed through `x-headroom-base-url`,
so the proxy can forward to the real provider without changing OpenCode
config.
- External `http2.connect` is blocked loudly instead of allowing direct
provider traffic to leak outside Headroom.

### Live provider additions

Provider coverage is no longer based on a static config scan. Because
routing happens at outbound request time, providers added mid-session
are routed through Headroom automatically as long as they use the
covered Node transport paths.

### Subagent and child-process coverage

- The parent OpenCode plugin sets a packaged Node preload shim through
`NODE_OPTIONS=--import=.../hook-shim/handler.js`.
- The transport shim patches `child_process.spawn`, `exec`, `execFile`,
and `fork` so child Node processes receive the Headroom preload even
when OpenCode passes a custom `env`.
- The child-process shim fails closed if it loads without
`HEADROOM_OPENCODE_TRANSPORT_PROXY_URL`.
- This closes the subagent leak path where a child Node process could
otherwise start without Headroom transport interception.

## Why this goes beyond PR #1089

PR #1089 improves OpenCode provider registration, but it still focuses
on provider config shape. This PR moves the enforcement boundary to
runtime transport interception.

This PR goes further because:

- No provider URL rewriting is required.
- New providers added mid-session are covered automatically.
- Subagents and child Node processes inherit the Headroom transport
shim.
- Direct external HTTP/2 paths fail loudly instead of leaking.
- The wrap remains transparent to the user's OpenCode provider config.
- The wrapper is fail-closed for unsupported child-process preload
state.

## Additional robustness fixes

While validating the change in Docker, the full Python suite exposed
unrelated Linux/container robustness issues. These are fixed in this PR
so the suite is green:

- Binary cache handling now treats cache paths under a non-writable
existing parent as unavailable, including when tests run as root in
Docker.
- `release_version.py` honors `MANUAL_VER` before git calls so direct
script execution works outside a `.git` checkout.
- Test logger isolation now resets relevant Headroom child loggers so
proxy logging setup cannot poison later `caplog` tests.
- The scanner missing-path test now uses a guaranteed missing `tmp_path`
child instead of relying on `/nonexistent/path`.

## Validation

All implementation validation was run inside Docker.

- Full Python suite from a fresh Docker copy: `6605 passed, 523
skipped`.
- Ruff on changed Python/OpenCode paths: passed.
- OpenCode plugin typecheck: passed.
- OpenCode plugin tests: `9 passed`.
- OpenCode plugin build: passed.
- Hook shim preload smoke test: passed.

## Notes

This PR intentionally does not add a CLI option. `headroom wrap
opencode` means full wrap. Either Headroom wraps OpenCode transparently,
or the path fails loudly instead of silently leaking provider traffic.

---------

Co-authored-by: Rudimar Ronsoni <6081613+rudironsoni@users.noreply.github.com>

plugins/opencode/package.json

@@ -0,0 +1,46 @@
+{
+  "name": "headroom-opencode",
+  "version": "0.1.0",
+  "description": "Headroom proxy integration plugin for OpenCode - routes LLM traffic through the Headroom proxy for token compression",
+  "type": "module",
+  "main": "./dist/index.js",
+  "types": "./dist/index.d.ts",
+  "files": [
+    "dist",
+    "hook-shim",
+    "README.md"
+  ],
+  "scripts": {
+    "build": "tsup",
+    "test": "vitest run",
+    "test:watch": "vitest",
+    "typecheck": "tsc --noEmit"
+  },
+  "dependencies": {
+    "@opencode-ai/plugin": "^1.17.8",
+    "headroom-ai": "^0.22.3"
+  },
+  "peerDependencies": {
+    "@ai-sdk/openai-compatible": "*",
+    "@ai-sdk/provider": "*",
+    "ai": "*"
+  },
+  "peerDependenciesMeta": {
+    "@ai-sdk/openai-compatible": {
+      "optional": true
+    },
+    "@ai-sdk/provider": {
+      "optional": true
+    },
+    "ai": {
+      "optional": true
+    }
+  },
+  "devDependencies": {
+    "@types/node": "^22.10.0",
+    "tsup": "^8.0.0",
+    "typescript": "^5.5.0",
+    "vitest": "^4.1.5"
+  },
+  "license": "Apache-2.0"
+}