feat: improve line prefix display with box drawing character

[LinruoT]

Summary

• Replace tab character with box drawing character │ for line number prefix
• Update regex pattern to handle new prefix format

Motivation

DeepSeekV4 often confuses the tab separator in line number prefixes with actual tab characters in file content when updating code files, leading to update errors. Using │ as the line number prefix separator eliminates this confusion.

[github-actions]

PR quality triage

Changed areas: area:cli-core

CLI core policy: Blocked by policy until a maintainer applies allow-cli-core-change and approves the PR.

CLI core files:

• src/tools/FileEditTool/prompt.ts
• src/utils/file.ts

Expected checks:

• change-policy
• server-checks

Test coverage signals:

• Agent/model runtime path changed: use mock/request-shape tests in PR and maintainer live-model smoke before release.

Risk notes:

• No special risk notes from changed paths.

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