feat: add walkthrough artifacts when agent has completed task implementation

[leslieo2]

• Added “generate walkthrough” flow: prompt wiring, LiveKit background job, DSML sanitization, and VS Code host file-existence check to create/update .pochi/walkthroughs/.md.
• Frontend tools UI now surfaces a walkthrough button on completion, checks for existing walkthroughs, and lets users open or regenerate walkthrough notes.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 2.88809% with 269 lines in your changes missing coverage. Please review.
✅ Project coverage is 20.42%. Comparing base (7b5695d) to head (d19a68e).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...c/features/tools/components/attempt-completion.tsx	0.00%	191 Missing ⚠️
...s/tools/components/new-task/subagent-renderers.tsx	2.38%	41 Missing ⚠️
...scode/src/integrations/webview/vscode-host-impl.ts	0.00%	15 Missing ⚠️
...i/src/features/tools/components/new-task/index.tsx	0.00%	11 Missing ⚠️
...code-webui/src/components/message/message-list.tsx	0.00%	4 Missing ⚠️
...code-webui/src/features/tools/components/index.tsx	0.00%	3 Missing ⚠️
...e-webui/src/features/chat/components/chat-area.tsx	0.00%	2 Missing ⚠️
packages/vscode-webui/src/features/chat/page.tsx	0.00%	1 Missing ⚠️
packages/vscode-webui/src/lib/vscode.ts	0.00%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (2.88%) is below the target coverage (75.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #895      +/-   ##
==========================================
+ Coverage   16.14%   20.42%   +4.28%     
==========================================
  Files         566      566              
  Lines       51743    51992     +249     
  Branches     1684     1332     -352     
==========================================
+ Hits         8354    10621    +2267     
+ Misses      43158    41144    -2014     
+ Partials      231      227       -4     

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