docs: 补充 Linux 打包完整步骤（sidecar 构建 + 前端构建 + electron-builder）

[njjtan]

之前的 README 只写了最后一步 electron-builder 命令，
缺少 build:sidecars 和 build:electron 两个前置步骤，
导致打包后运行报错 'sidecar binary not found'。

补充内容：

• 完整的 4 步打包流程（install → sidecar → electron → package）
• deb 安装和 AppImage 运行方法
• 产物路径说明
• 清理旧产物提示

Summary

Feature Quality Contract

• Changed surface:
• Tests added or updated:
• Coverage evidence:
• E2E / live-model evidence:
• Known risk / rollback:

Verification

• I ran the relevant local checks, or explained why they do not apply.
• I added or updated same-area tests for every production behavior change.
• I ran bun run verify for code changes, including the coverage gate.
• New or changed executable production lines meet the changed-line coverage threshold, or the blocker/maintainer override is documented.
• I attached or summarized the quality report path, JUnit/log artifact path, and pass/fail/skip counts.
• I ran E2E/live smoke for cross-boundary, provider/runtime, desktop chat, agent-loop, native, or release changes, or documented the blocker.

Risk

• This PR does not touch CLI core paths, or it has maintainer approval for allow-cli-core-change.
• Production code changes include matching tests, or have maintainer approval for allow-missing-tests.
• Coverage baseline/threshold changes have maintainer approval for allow-coverage-baseline-change.
• Quarantined tests still have owners, exit criteria, and unexpired review windows.
• Provider/runtime changes were covered by mock contract tests, and live smoke was run or explicitly deferred.

@dosubot review this PR for changed-area risk, missing tests, docs impact, desktop startup risk, and CLI core impact.

[github-actions]

PR quality triage

Changed areas: area:docs

CLI core policy: No CLI-core policy block detected.

Missing-test policy: No missing-test policy block detected.

Coverage baseline policy: No coverage-baseline policy block detected.

CLI core files:

• none

Coverage policy files:

• none

Expected checks:

• change-policy
• docs-checks

Test coverage signals:

• No obvious missing-test signal from changed paths.

Risk notes:

• No special risk notes from changed paths.

Hard merge gates still come from GitHub Actions, not AI review.

Dosu handoff: Dosu can be used as the AI reviewer for risk explanation, missing-test prompts, and maintainer Q&A. If it does not comment automatically from the PR template, ask:

@dosubot review this PR for changed-area risk, missing tests, docs impact, desktop startup risk, and CLI core impact.