feat: merge multiple sessions into one, with similarity scoring

[LiuQhahah]

Closes #1163

Summary

This PR adds a session merge feature to the sidebar for the pi coding
agent web UI: pick two or more sessions, preview how similar they are, and
combine them into one session file (sources are deleted afterwards).

User story

As a pi user working across several related sessions of the same project,
I want to merge multiple sessions into a single one, so that I can keep one
coherent history instead of a fragmented list — and before anything is
deleted, I want to know how similar the sessions are, so I only merge
ones that actually belong together.

What it does

• Selection: row menu Merge… (also bound to M) and a top-bar merge
  button open a searchable multi-select dialog.
• Similarity preview: every pair of selected sessions is scored by
  Dice coefficient over shingles of the actual .jsonl content
  (thresholds: ≥ 0.45 suggests merging, < 0.25 warns). The UI shows the
  per-pair score and an overall verdict.
• Safety gates:
  • Dissimilar pairs need an explicit “merge anyway” confirmation
    (force); the server re-validates it (409 on refusal).
  • Default same-directory only: sessions whose header cwd differs
    are refused unless the user turns off “only merge sessions in the same
    directory”; the server enforces this (sameCwdOnly, 400 on refusal).
  • Subagent sessions are refused — their pi-web:subagent metadata
    cannot be carried into a normal session (409).
• Merge semantics:
  • The first selected session is the main line; following sessions have
    their ids re-mapped and the parent/child tree relinked (branch
    structure preserved), then are appended after the main line with a
    custom_message source marker.
  • Sources are deleted after success; existing fork children are
    re-parented to the new file. The merged session is created beside the
    main session and opened in the UI.
• i18n: en / zh-CN / zh-TW.

Files

• lib/session-merge.ts — scoring (textShingles, dice),
  previewSessionMerge, mergeSessions, SessionMergeError
• app/api/sessions/merge/route.ts — POST endpoint (preview or execute)
• components/SessionMergeDialog.tsx — the dialog
• components/SessionSidebar.tsx, components/SidebarIcons.tsx,
  lib/sidebar-actions.ts — entry points
• app/sidebar.css — dialog styling
• lib/session-merge.test.mjs — 9 unit tests (node:test + jiti)

Test status

• node --experimental-strip-types --test lib/session-merge.test.mjs — 9/9
• tsc --noEmit — clean