Comment template engine for PR comments (#1)

* comment template engine for PR comments

Ports the dashboard's Handlebars PR comment templates to Go templates
with a structured Data/Inputs pipeline. All comment sections are
implemented: errors, governance (FinOps, security, tagging, guardrails),
fixed issues, cost change sentence, carbon/water summary, project costs
table, cost details diff output, preexisting issues, and truncation.

Depends on:
- infracost/go-proto#10 (GuardrailName)
- infracost/proto#43 (FinopsPolicyFailingResource source location)

* use actual libs

Author: liamcervante

README.md

@@ -1 +1,76 @@
-# vcs
+# vcs
+
+Go library for posting Infracost cost estimate comments to pull requests. Supports GitHub (including Enterprise), with a provider interface for adding GitLab, Bitbucket, Azure Repos, etc.
+
+## Usage
+
+### 1. Create a provider
+
+```go
+gh, err := github.New(ctx, "my-org", "my-repo", token, prNumber, github.Options{})
+```
+
+For GitHub Enterprise, set `Options.APIURL` to your instance URL.
+
+### 2. Build the comment data
+
+Populate a `vcs.CommentData` struct with cost and policy data from your run. All fields use shared proto types from `github.com/infracost/proto`.
+
+```go
+data := vcs.CommentData{
+    Currency:             "USD",
+    TotalMonthlyCost:     totalCost,
+    PastTotalMonthlyCost: pastCost,
+    Projects: []vcs.ProjectResult{
+        {
+            Name:             "my-project",
+            TotalMonthlyCost: projectCost,
+            Resources:        output.GetResources(),
+            DiffResources:    diffResources,
+        },
+    },
+}
+```
+
+### 3. Generate and post the comment
+
+```go
+body, err := gh.GenerateComment(data)
+result, err := gh.PostComment(ctx, body, vcs.BehaviorUpdate)
+```
+
+## Comment behaviors
+
+`PostComment` accepts a `vcs.Behavior` that controls how comments are managed:
+
+| Behavior | Description |
+|---|---|
+| `BehaviorUpdate` | Find the existing Infracost comment and update it, or create one if none exists. Skips if the existing comment is newer or identical. |
+| `BehaviorNew` | Always create a new comment. |
+| `BehaviorHideAndNew` | Minimize all existing Infracost comments, then create a new one. |
+| `BehaviorDeleteAndNew` | Delete all existing Infracost comments, then create a new one. |
+
+Comments are identified by an invisible markdown tag (`[//]: <> (infracost-comment)`) embedded at the top of the comment body. This tag is added automatically by `PostComment` and includes a `valid-at` timestamp to prevent race conditions between concurrent runs.
+
+## Custom templates
+
+Each provider uses a default HTML/Markdown template. To override it, pass a custom `*template.Template` via the provider options:
+
+```go
+gh, err := github.New(ctx, owner, repo, token, pr, github.Options{
+    Template: myTemplate,
+})
+```
+
+The template receives a `vcs.CommentData` struct and has access to the template functions defined in `template_funcs.go`.
+
+## Interface
+
+Any VCS provider implements:
+
+```go
+type VCS interface {
+    GenerateComment(CommentData) (string, error)
+    PostComment(ctx context.Context, body string, behavior Behavior) (PostResult, error)
+}
+```

go.mod

@@ -0,0 +1,33 @@
+module github.com/infracost/vcs
+
+go 1.25.5
+
+require (
+	github.com/google/go-cmp v0.7.0
+	github.com/infracost/proto v1.27.0
+	github.com/shurcooL/githubv4 v0.0.0-20260209031235-2402fdf4a9ed
+	golang.org/x/oauth2 v0.36.0
+)
+
+require (
+	github.com/agext/levenshtein v1.2.1 // indirect
+	github.com/agnivade/levenshtein v1.2.1 // indirect
+	github.com/apparentlymart/go-textseg/v15 v15.0.0 // indirect
+	github.com/hashicorp/hcl/v2 v2.24.0 // indirect
+	github.com/mitchellh/go-wordwrap v1.0.1 // indirect
+	github.com/shurcooL/graphql v0.0.0-20240915155400-7ee5256398cf // indirect
+	github.com/zclconf/go-cty v1.17.0 // indirect
+	golang.org/x/mod v0.30.0 // indirect
+	golang.org/x/sync v0.19.0 // indirect
+	golang.org/x/tools v0.39.0 // indirect
+)
+
+require (
+	github.com/infracost/go-proto v0.8.0
+	golang.org/x/net v0.48.0 // indirect
+	golang.org/x/sys v0.39.0 // indirect
+	golang.org/x/text v0.32.0 // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20251202230838-ff82c1b0f217 // indirect
+	google.golang.org/grpc v1.79.3 // indirect
+	google.golang.org/protobuf v1.36.11 // indirect
+)

go.sum

@@ -0,0 +1,78 @@
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