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"
```

scripts/check_emdash.sh

@@ -20,9 +20,14 @@ emdash=$'\xE2\x80\x94'
 endash=$'\xE2\x80\x93'
 pattern="${emdash}|${endash}"
 
+# Git exclude_pathspecs excluded from the check. Used in both ls-files and diff comparison.
+exclude_pathspecs=(
+	":(exclude)aibridge/fixtures/**/*.txtar"
+)
+
 scan_all_files() {
 	local output
-	output=$(git ls-files -z | xargs -0 grep -IEn "$pattern" 2>/dev/null || true)
+	output=$(git ls-files -z -- "${exclude_pathspecs[@]}" | xargs -0 grep -IEn "$pattern" 2>/dev/null || true)
 	if [[ -n "$output" ]]; then
 		echo "$output"
 		found=1
@@ -58,7 +63,7 @@ else
 		fi
 
 		found=0
-		if ! diff_output=$(git diff "$base" -U0 -- . 2>&1); then
+		if ! diff_output=$(git diff "$base" -U0 -- . "${exclude_pathspecs[@]}" 2>&1); then
 			echo "ERROR: git diff against $base failed:"
 			echo "$diff_output"
 			exit 1