fix(image-paste): clear draft attachments on session start

[outlier27-cell]

Problem

Image-paste draft placeholder mappings lived until session_shutdown, although Pi can begin /new, /resume, and /reload sessions through session_start. A new Session could expand [Image #1] to a previous Session's temporary path.

Closes #742

Value

Clipboard-image draft state is now correctly scoped to one Pi Session.

Approach

• Clear the image attachment store before each new session installs its editor layer.
• Add an extension lifecycle regression proving a stale placeholder remains literal after session_start.

Validation

• node --test --experimental-strip-types tests/extensions/image-paste/index.test.ts (36 passing)
• bun run check
• The full suite has known failures in unrelated renderer, Git fixture, and clear-context coverage on this baseline; exact-head GitHub CI is tracked for the final cross-platform result.

Impact

Existing submission, dequeue, and history-recall behavior stays unchanged within a Session. Only stale mappings are discarded at its boundary.

[outlier27-cell]

@tt-a1i 请审核此 PR。当前精确提交的 GitHub CI 已全部通过，修改范围与回归测试均已在 PR 说明中列明；现仅等待维护者审核。

[outlier27-cell]

@testikun @tt-a1i 请审核此 PR。最新提交的 CI 已通过，当前仅等待维护者审核与合并。

[tt-a1i]

Reviewed head: 284d48ec2b9a8b20c6e046a50d392879aa2456b1 against main a18f457687b45c877bc8a15d689a7580c4f0659a.

Standards: no confirmed code-level blocker in the scoped change.

Spec: the inspected change matches the stated behavior in this PR. #743 and #688 overlap in Session draft cleanup; retain one integrated behavior and both relevant regressions.

Validation: node --test --experimental-strip-types tests/extensions/image-paste/index.test.ts: exit=0; ℹ tests 36; ℹ pass 36; ℹ fail 0; ℹ skipped 0

The unchanged-head focused receipts are reused after a live head/main refresh; #718 uses this round's latest-head tests. This code review does not claim installed-runtime, live-provider, full merged-queue or visual acceptance. No merge is authorized by this review.