feat: add flashduty-statuspage agent skill

[debidong]

Summary

Adds the tenth Flashduty agent skill, covering the statuspage command family (4 existing operations + 4 migration subcommands) for AI coding agents.

Changes

• NEW skills/flashduty-statuspage/SKILL.md (~264 lines). Inline command sections matching the flashduty-channel / flashduty-insight structural pattern — no references/ subdirectory.
• README.md: "ships with 9 agent skills" → "10"; added skill row.
• README_zh.md: same drift fixed in zh copy.

Skill scope

Covers both day-to-day statuspage operations and the one-time Atlassian → Flashduty migration:

Command	Purpose
`statuspage list`	Find page IDs
`statuspage changes`	List active incidents / maintenance on a page
`statuspage create-incident`	Publish a new incident
`statuspage create-timeline`	Post timeline updates / drive status transitions
`statuspage migrate structure`	Start structure + history migration (async)
`statuspage migrate email-subscribers`	Start subscriber migration (async, separate job)
`statuspage migrate status`	Poll migration job progress
`statuspage migrate cancel`	Cancel in-flight migration

Three workflows documented: incident publication lifecycle, full Atlassian migration (structure → verify → subscribers), and migration cancel.

Triggering scope

Description narrowly scoped to status page management + migration. Does NOT claim coverage of internal incident response (that's `flashduty-incident`) or channel/escalation setup (that's `flashduty-channel`). Cross-references both as related skills.

Verification

• Flag tables verified against live CLI help for all 8 subcommands
• Component status enumerations match `create-incident` / `create-timeline` help text
• Cross-references resolve (`flashduty-shared`, `flashduty-incident`, `flashduty-channel`)
• Source provider claim (`atlassian` only) verified against `internal/cli/status_page_migrate.go`:10 + `validateMigrationSource` at :168
• Frontmatter matches other skills (`name`, `version: 1.0.0`, `metadata.requires.bins: ["flashduty"]`, `metadata.cliHelp`)
• Structural sections match `flashduty-channel` / `flashduty-insight` ordering (Overview → Quick Decision → Commands → Workflows → Key Concepts → Cross-References)
• Size (264 lines) in range with other skills (100–265)
• Independent audit agent reported no real defects
• `gofmt -l .`, `go vet ./...`, `go test -race ./...` — no changes to code, all green

Non-goals

• Not regenerating other skills' docs.
• Not updating installer manifests — skill discovery is filesystem-based per existing convention.
• Not adding `references/` subdirectory — inline chosen per 8 of 9 existing skills.