Add GitHub issue as prompt input

prompt.md

@@ -0,0 +1,165 @@
+# Plan: GitHub Issue as Prompt Input
+
+Support starting a ralph loop from a GitHub issue number or URL. The issue becomes the prompt.md content.
+
+## Overview
+
+User provides a GitHub issue reference (number like `42` or URL like `https://github.com/owner/repo/issues/42`) and ralph fetches the issue, generates a prompt.md, and starts the loop.
+
+## Input Formats
+
+Accept these input patterns:
+- Issue number: `42`, `#42`
+- Full URL: `https://github.com/owner/repo/issues/42`
+- Short reference: `owner/repo#42`
+
+## Implementation
+
+### 1. New Package: `internal/github/issue.go`
+
+Create functions to:
+- Parse issue references into owner/repo/number
+- Fetch issue via `gh issue view <number> --json title,body,labels,assignees,milestone`
+- Return structured `Issue` type with title, body, labels, etc.
+
+```go
+type Issue struct {
+    Number    int
+    Title     string
+    Body      string
+    Labels    []string
+    URL       string
+    Assignees []string
+    Milestone string
+}
+
+func ParseIssueRef(ref string) (owner, repo string, number int, err error)
+func FetchIssue(number int) (*Issue, error)
+```
+
+### 2. Prompt Generation: `internal/github/prompt.go`
+
+Convert issue to prompt.md content:
+
+```markdown
+# Issue #42: <title>
+
+<issue body>
+
+---
+Source: <issue URL>
+Labels: <comma-separated labels>
+```
+
+Function signature:
+```go
+func GeneratePrompt(issue *Issue) string
+```
+
+### 3. CLI Changes: `cmd/ralph/main.go`
+
+Add new flag:
+- `-i, --issue STRING` - GitHub issue number or URL
+
+Add logic in PreRunE or RunE:
+- If `--issue` provided and no `-p` prompt file:
+  - Parse issue reference
+  - Fetch issue details
+  - Generate prompt content
+  - Write to `prompt.md` (or temp file)
+  - Set prompt path to generated file
+
+### 4. Config Changes: `internal/config/config.go`
+
+Add to Config struct:
+```go
+Issue string `yaml:"issue"` // GitHub issue reference
+```
+
+Add to flag binding and YAML loading.
+
+### 5. Worktree Integration
+
+When `--issue` is used with `-w` worktree mode:
+- Auto-generate branch name from issue: `ralph/issue-42-short-title`
+- Use issue number in worktree path
+
+Modify `internal/worktree/worktree.go`:
+```go
+func BranchNameFromIssue(issue *Issue) string
+```
+
+### 6. PR Linking
+
+When creating PR with `--pr` after working on an issue:
+- Include `Fixes #42` or `Closes #42` in PR body
+- Reference the original issue URL
+
+Modify `internal/runner/runner.go` in `runPRPhase`:
+- Check if session started from issue
+- Add issue reference to PR body template
+
+### 7. Issue Comment (Optional Enhancement)
+
+Post a comment on the issue when work starts/completes:
+- `gh issue comment <number> --body "Started working on this issue..."`
+- Include branch name, PR link when done
+
+New functions in `internal/github/`:
+```go
+func CommentOnIssue(number int, body string) error
+func LinkPRToIssue(issueNumber, prNumber int) error
+```
+
+## File Changes Summary
+
+| File | Change |
+|------|--------|
+| `cmd/ralph/main.go` | Add `--issue` flag, issue fetching logic |
+| `internal/config/config.go` | Add `Issue` field to Config |
+| `internal/github/issue.go` | New - issue parsing and fetching |
+| `internal/github/prompt.go` | New - prompt generation from issue |
+| `internal/worktree/worktree.go` | Add `BranchNameFromIssue()` |
+| `internal/runner/runner.go` | Pass issue context, modify PR body |
+| `internal/git/pr.go` | Add issue reference to PR body |
+
+## Usage Examples
+
+```bash
+# Start loop from issue number (uses current repo)
+ralph -i 42
+
+# Start loop from issue URL
+ralph -i https://github.com/owner/repo/issues/42
+
+# With worktree (auto-names branch from issue)
+ralph -i 42 -w
+
+# Full workflow: issue -> worktree -> PR
+ralph -i 42 -w --pr
+
+# With iteration limits
+ralph -i 42 -n 10 -t 30m
+```
+
+## Testing
+
+- Unit tests for issue reference parsing (various formats)
+- Unit tests for prompt generation
+- Integration test with mock `gh` output
+- Add test scenario to `internal/testmode/` for issue-based runs
+
+## Error Handling
+
+- `gh` CLI not installed: clear error message with install link
+- Issue not found: "Issue #42 not found in owner/repo"
+- No repo context: "Cannot determine repository. Use full URL or run from a git repo"
+- Rate limiting: surface GitHub API errors clearly
+
+## Future Enhancements
+
+- Support issue templates for prompt formatting
+- Fetch issue comments for additional context
+- Support GitLab/other forges via adapter pattern
+- Auto-assign issue when starting work
+- Update issue labels (e.g., add "in-progress")