Rearchitect background todo agent

[vritant24]

• Less complex implementation
• simple exclusion of a set of Tools, others have their arguments trimmed to a limit of characters
• every successive run of the BG Todo Agent runs after a longer wait. This means longer sessions have less frequent runs of the todo agent.
• Once a turn ends in the conversation, the bg todo agent runs, and for a continuation, clears the existing todos.
• previous turn tool calls are not carried, only the previous todos and their states are.

[Copilot]

Pull request overview

This PR re-architects the Copilot extension’s background todo agent by removing the previous delta-tracker/state-machine implementation and replacing it with a queued, per-turn processor plus a dedicated turn-history store and updated prompt format.

Changes:

• Replaced the legacy BackgroundTodoProcessor/delta tracker with a new BackgroundTodoAgentProcessor that queues per-turn work and performs a final pass at turn end.
• Introduced BackgroundTodoAgentSessionHistoryStore to accumulate and classify tool-call rounds and drive backoff readiness.
• Updated todo prompt + intent wiring, and added a clearCurrentTodoContext(...) API to reset the todo widget at the start of a turn.

Show a summary per file

File	Description
extensions/copilot/src/extension/prompts/node/agent/backgroundTodoProcessor.ts	Deleted legacy background todo processor state machine, policy, prompt history rendering, and execution logic.
extensions/copilot/src/extension/prompts/node/agent/backgroundTodoDelta.ts	Deleted legacy delta tracker used to compute “new activity since last pass.”
extensions/copilot/src/extension/prompts/node/agent/backgroundTodoAgent/processor.ts	Added new queued background todo processor with progressive backoff and per-turn lifecycle (track rounds + endTurn final pass).
extensions/copilot/src/extension/prompts/node/agent/backgroundTodoAgent/historyStore.ts	Added per-turn tool-round tracking, filtering/classification, and “unprocessed substantive round” counting.
extensions/copilot/src/extension/prompts/node/agent/backgroundTodoAgent/backgroundTodoAgentPrompt.tsx	Updated prompt format and rendering to use the new history store and include previous-turn todos + a final-review note.
extensions/copilot/src/extension/prompt/node/todoListContextProvider.ts	Added clearCurrentTodoContext(...) to reset the todo list via the manage_todo_list tool.
extensions/copilot/src/extension/intents/node/agentIntent.ts	Wired the new processor into the main agent flow (track per render + run endTurn final pass).

Copilot's findings

Comments suppressed due to low confidence (2)

extensions/copilot/src/extension/prompts/node/agent/backgroundTodoAgent/backgroundTodoAgentPrompt.tsx:197

• This comment says each chunk wraps the round in <turn> tags, but the implementation now only emits a <round> block. This mismatch makes it harder to understand pruning/tag-balance expectations.
  extensions/copilot/src/extension/prompts/node/agent/backgroundTodoAgent/backgroundTodoAgentPrompt.tsx:143
• userRequest is typed as string | undefined and is rendered directly. If it’s ever undefined (e.g. unexpected call ordering), the prompt will literally include “undefined”, which can degrade tool-call behavior.

• Files reviewed: 7/7 changed files
• Comments generated: 6

[Copilot]

Copilot's findings

Comments suppressed due to low confidence (3)

extensions/copilot/src/extension/prompts/node/agent/backgroundTodoAgent/backgroundTodoAgentPrompt.tsx:197

• This comment says each chunk wraps its round in <turn> tags to keep tags balanced after pruning, but the implementation no longer emits <turn> wrappers (it only renders <round>). The comment is now misleading and should be updated to match the actual rendering strategy.
  extensions/copilot/src/extension/prompts/node/agent/backgroundTodoAgent/backgroundTodoAgentPrompt.tsx:18
• The system message says the <tool-calls> list includes a file-path/category target and optional intent note, but the renderer actually outputs "Tool Call Name:" + "Arguments:" lines (no extracted target/note). This mismatch can confuse the model and reduce tool-call parsing reliability.
  extensions/copilot/src/extension/prompts/node/agent/backgroundTodoAgent/backgroundTodoAgentPrompt.tsx:143
• userRequest is typed as optional but is rendered unconditionally. If it is ever unset, the prompt will literally contain undefined, which is noise for the model and could affect behavior. Either make the prop required or guard the render.

• Files reviewed: 15/15 changed files
• Comments generated: 5