github: support issues and workflow_dispatch events

[elithrar]

Adds support for issues (opened, edited) and workflow_dispatch events in the GitHub Action. Picks up from the work in #5810.

I've used this as an opportunity to refactor the event handling to make it easier to read + maintain going forward.

• categorize events into USER_EVENTS and REPO_EVENTS for clearer routing
• USER_EVENTS (issue_comment, pull_request_review_comment, issues, pull_request): have actor, support reactions/comments
• REPO_EVENTS (schedule, workflow_dispatch): no actor, output to logs/PR only
• require prompt input for issues events (no comment to parse)
• add reaction to issue itself for issues events (same as pull_request)
• update docs with new events in supported events table
• add issues triage workflow example with account age filter
• fix docs formatting for scheduled workflows note

Event Routing

┌─────────────────────────────────────────────────────────────────────────────┐
│                           GithubRunCommand.handler()                        │
│  1. Parse context & normalize inputs                                        │
│  2. Get app token (OIDC or GITHUB_TOKEN)                                    │
│  3. Configure git                                                           │
└─────────────────────────────────────────────────────────────────────────────┘
                                      │
                    ┌─────────────────┴─────────────────┐
               isUserEvent                        isRepoEvent
                    │                                   │
                    ▼                                   ▼
┌───────────────────────────────────┐  ┌───────────────────────────────────┐
│         USER_EVENTS               │  │         REPO_EVENTS               │
│  - issue_comment                  │  │  - schedule                       │
│  - pull_request_review_comment    │  │  - workflow_dispatch              │
│  - issues (opened, edited)        │  │                                   │
│  - pull_request                   │  │                                   │
├───────────────────────────────────┤  ├───────────────────────────────────┤
│  FLOW:                            │  │  FLOW:                            │
│  1. assertPermissions(actor)      │  │  1. (no permission check)         │
│  2. addReaction()                 │  │  2. (no reaction)                 │
│  3. chat() with context           │  │  3. chat()                        │
│  4. createComment(response)       │  │  4. console.log() or createPR()   │
│  5. removeReaction()              │  │                                   │
└───────────────────────────────────┘  └───────────────────────────────────┘

USER vs. REPO event handling

Aspect	USER_EVENTS	REPO_EVENTS
Actor	Present	None
Issue/PR context	Always has issueId	None
Permission check	Yes	No
Reaction	Add before, remove after	None
Output	Comment on issue/PR	Logs only (+ PR if changes)
Prompt source	Comment body or PROMPT env	PROMPT env (required)