feat: add shareable GitHub handoff templates

[Yuki9814]

Maintainer problem

PatchHive currently exports one complete internal mission record. That is useful for audit, but maintainers still have to manually trim workflow stages, agent drafts, approvals, and unrelated evidence before pasting a handoff into a GitHub Issue or Pull Request.

Change

• add Full evidence handoff, GitHub Issue, and GitHub Pull Request formats
• keep preview, privacy preflight, clipboard, download, status messages, and filenames on the same selected Markdown payload
• retain mapped handoff evidence, accepted scanner risks, and complete-rerun provenance in compact GitHub templates
• omit unmapped evidence and internal workflow bookkeeping from external templates
• keep the format choice ephemeral: no workspace schema or storage migration
• update v0.6.0 product documentation and roadmap

Maintainer impact

Maintainers can produce review-ready GitHub Markdown without authentication, a backend, automatic posting, or expanding PatchHive's network boundary. The existing full evidence export remains the default.

Evidence and verification

• lint passed
• 136 unit/component tests passed
• production TypeScript/Vite build and Pages path smoke passed
• dependency audit found 0 vulnerabilities
• 13 Chromium E2E workflows passed
• 25/100/250-finding JSON and SARIF benchmark passed on Apple M5 / 16 GB
• desktop 1280x720 and mobile 390x820 browser QA passed with no horizontal overflow or console warnings/errors
• privacy preflight verified against the selected Pull Request template
• git diff --check passed

Risk and rollback

The new selector is local ephemeral UI state and does not alter schema v8, import/export compatibility, storage, CSP, or network behavior. Reverting this commit restores the single full-handoff format.