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"
```

.github/workflows/ci.yaml

@@ -80,6 +80,7 @@ jobs:
               # Main repo directories for completeness in case other files are
               # touched:
               - "agent/**"
+              - "aibridge/**"
               - "cli/**"
               - "cmd/**"
               - "coderd/**"

.github/workflows/typos.toml

@@ -57,4 +57,6 @@ extend-exclude = [
 	# notifications' golden files confuse the detector because of quoted-printable encoding
 	"coderd/notifications/testdata/**",
 	"agent/agentcontainers/testdata/devcontainercli/**",
+	# aibridge fixtures contain truncated streaming chunks that look like typos
+	"aibridge/fixtures/**",
 ]

.github/zizmor.yml

@@ -1,7 +1,7 @@
 rules:
   cache-poisoning:
     ignore:
-      - "ci.yaml:187"
+      - "ci.yaml:188"
   dangerous-triggers:
     ignore:
       # Both workflows use pull_request_target intentionally: they need