feat: upload job summary when supported (#917)

- Add GitHub-style Actions **job summaries** support (writes to `GITHUB_STEP_SUMMARY` / `workflow/SUMMARY.md`) and render them in the run UI.
- Gitea stores summaries internally (DB) and serves them in the run view payload.
- `act_runner` uploads the summary **only when Gitea advertises support** (`X-Gitea-Actions-Capabilities: job-summary`), and warns on upload failures without failing the job.

## Compatibility
- New Gitea + old runner: no upload → no summary shown (no behavior change)
- New runner + old Gitea: capability not advertised → runner skips upload (no behavior change)

## Issue
- Fixes go-gitea/gitea#23721

Reviewed-on: https://gitea.com/gitea/runner/pulls/917
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: Zettat123 <39446+zettat123@noreply.gitea.com>

act/runner/run_context.go

@@ -36,15 +36,19 @@ import (
 
 // RunContext contains info about current job
 type RunContext struct {
-	Name                string
-	Config              *Config
-	Matrix              map[string]any
-	Run                 *model.Run
-	EventJSON           string
-	Env                 map[string]string
-	GlobalEnv           map[string]string // to pass env changes of GITHUB_ENV and set-env correctly, due to dirty Env field
-	ExtraPath           []string
-	CurrentStep         string
+	Name        string
+	Config      *Config
+	Matrix      map[string]any
+	Run         *model.Run
+	EventJSON   string
+	Env         map[string]string
+	GlobalEnv   map[string]string // to pass env changes of GITHUB_ENV and set-env correctly, due to dirty Env field
+	ExtraPath   []string
+	CurrentStep string
+	// CurrentStepIndex is the index of the top-level job step currently executing
+	// (model.Step.Number). Composite sub-steps inherit the outer step's index by
+	// walking the Parent chain; see topLevelRunContext.
+	CurrentStepIndex    int
 	StepResults         map[string]*model.StepResult
 	IntraActionState    map[string]map[string]string
 	ExprEval            ExpressionEvaluator
@@ -57,6 +61,14 @@ type RunContext struct {
 	Masks               []string
 	cleanUpJobContainer common.Executor
 	caller              *caller // job calling this RunContext (reusable workflows)
+	// summaryFileInitialized tracks which per-step summary files (workflow/step-summary-N.md)
+	// have already been created on the JobContainer. The runner sets up file-command files
+	// via JobContainer.Copy at the start of every phase, which truncates them — fine for
+	// GITHUB_ENV/OUTPUT/STATE/PATH (consumed per phase) but wrong for GITHUB_STEP_SUMMARY,
+	// which has accumulating semantics. We initialize each step's summary file exactly once
+	// so writes from later phases and from composite sub-steps append to the same file.
+	// Only populated on the top-level RunContext; child RCs walk Parent via topLevelRunContext.
+	summaryFileInitialized map[int]bool
 	// outputTemplate is this combination's pristine snapshot of the job's output expressions,
 	// captured before execution so each matrix combo interpolates from the originals rather
 	// than from a sibling's already-resolved values written into the shared Job.Outputs.
@@ -704,6 +716,17 @@ func (rc *RunContext) steps() []*model.Step {
 	return steps
 }
 
+// topLevelRunContext walks the Parent chain to the outermost RunContext. Composite
+// actions create child RunContexts whose sub-steps need to share the outer job step's
+// summary file path so that nested writes accumulate under the right step_index.
+func (rc *RunContext) topLevelRunContext() *RunContext {
+	top := rc
+	for top.Parent != nil {
+		top = top.Parent
+	}
+	return top
+}
+
 // Executor returns a pipeline executor for all the steps in the job
 func (rc *RunContext) Executor() (common.Executor, error) {
 	var executor common.Executor