feat(tfe): add list_run_comments tool (#347)

[ChrisJr404]

Closes #347.

`action_run` already lets agents post a run comment when applying / discarding / canceling, but there was no way to read the existing comments back. `get_run_details` returns the comment ID, but no body — so an agent inspecting a run can see that humans left context but can't act on it.

This wires the existing TFE Comments API (`GET /runs/{run_id}/comments`, surfaced via `go-tfe`'s `Comments.List`) as a new `list_run_comments` tool. Shape mirrors `list_runs`:

```json
// list_run_comments({"run_id": "run-CZcmD7eagjhyX0vN"})
{
"items": [
{"id": "comment-XXX", "body": "Approved by @sre, retrying after the 503"},
{"id": "comment-YYY", "body": "..."}
]
}
```

• Read-only and non-destructive annotations.
• Structured `RunCommentSummary` / `RunCommentList` types so future field additions (resolved-by, created-at) are a passthrough rather than a struct rewrite.

Wiring

Three files:

• New `pkg/tools/tfe/list_run_comments.go` (mirrors `list_runs.go` shape).
• `pkg/toolsets/mapping.go` — `list_run_comments → Terraform` toolset.
• `pkg/tools/dynamic_tool.go` — conditional registration alongside `list_runs`.

What's not in scope

• Pagination. The upstream `Comments.List` endpoint returns the full list in a single response (the go-tfe `CommentList` type embeds `*Pagination` but the endpoint doesn't actually populate page cursors). Output is wrapped in a struct anyway so pagination is a passthrough add later if upstream gets it.
• Read-by-id. `go-tfe` exposes `Comments.Read(commentID)` too, but the issue's ask is the list path. Happy to add a `get_run_comment` companion if reviewers want it.

Test plan

• `go build ./...` clean
• `go vet ./...` clean
• `go test ./pkg/tools/tfe/... ./pkg/toolsets/...` ok
• (For maintainer) — exercise against a real TFC org with a run that has 1+ comments and confirm the bodies render correctly. The go-tfe lib is already used heavily in this repo so the network path itself is well-trodden.

Note on duplication

@vavdoshka offered on the issue to contribute this — happy to defer if they were already partway through. Didn't see an open PR or a draft branch from them, so I went ahead. If you'd rather their PR land instead, totally fine to close this one.