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)

[rekram1-node]

/review

[github-actions]

lgtm