chore: move aibridge library code into coder repo (#24190)

This PR merges code from `coder/aibridge` repository into `coder/coder`.
It was split into 4 PRs for easier review but stacked PRs will need to
be merged into this PR so all checks pass.

* https://github.com/coder/coder/pull/24190 -> raw code copy (this PR,
before merging PRs on top of it, it was just 1 commit:
https://github.com/coder/coder/commit/70d33f33200c7e77df910957595715f81f9bec24)
* https://github.com/coder/coder/pull/24570 -> update imports in
`coder/coder` to use copied code
* https://github.com/coder/coder/pull/24586 -> linter fixes and CI
integration (also added README.md)
* https://github.com/coder/coder/pull/24571 -> added exclude to
scripts/check_emdash.sh check

Original PR message (before PR squash):
Moves coder/aibridge code into coder/coder repository.

Omitted files:

- `go.mod`, `go.sum`, `.gitignore`, `.github/workflows/ci.yml,`
`Makefile`, `LICENSE`, `README.md` (modified README.md is added later)
- `.github`, `example`, `buildinfo,` `scripts` directories

Simple verification script (will list omitted files)

```
tmp=$(mktemp -d)
echo "$tmp"
git clone --depth=1 https://github.com/coder/aibridge "$tmp/aibridge"
git clone --depth=1 --branch pb/aibridge-code-move https://github.com/coder/coder "$tmp/coder"
diff -rq --exclude=.git "$tmp/aibridge" "$tmp/coder/aibridge"
# rm -rf "$tmp"
```

.golangci.yaml

@@ -6,6 +6,21 @@ linters-settings:
     # goal: 100
     threshold: 412
 
+  depguard:
+    rules:
+      aibridge_import_isolation:
+        list-mode: lax
+        files:
+          - "aibridge/*.go"
+          - "aibridge/**/*.go"
+        allow:
+          - $gostd
+          - github.com/coder/coder/v2/aibridge
+          - github.com/coder/coder/v2/buildinfo
+        deny:
+          - pkg: github.com/coder/coder/v2
+            desc: aibridge code must not import coder packages outside aibridge; buildinfo is the only exception
+
   exhaustruct:
     include:
       # Gradually extend to cover more of the codebase.
@@ -227,6 +242,7 @@ linters:
     - asciicheck
     - bidichk
     - bodyclose
+    - depguard
     - dogsled
     - errcheck
     - errname