feat(agents): add built-in DeepResearch subagent

[bobleer]

Summary

• Adds DeepResearch as a built-in subagent (alongside Explore and FileFinder) using the Longitudinal + Cross-sectional Analysis method to produce comprehensive, sourced research reports
• Fixes a subagent deadlock bug where tool confirmation would block indefinitely since no user is present to respond
• Adds a capability profile for DeepResearch in the frontend agents UI

Changes

Core — new built-in agent

• deep_research_agent.rs: DeepResearchAgent struct with a curated tool set (WebSearch, WebFetch, Read, Grep, Glob, LS, Write, Bash, TerminalControl, TodoWrite). Task is explicitly excluded to prevent recursive subagent calls.
• deep_research_agent.md: system prompt implementing an incremental research workflow — orient → plan → research-and-write per chapter → synthesis — so context is never lost on long reports. Key constraints:
  • Every factual claim must be sourced, dated, or attributed; unsourced claims must be labeled (unverified) or (inferred)
  • Full report saved to {workspace}/deep-research/{slug}-{date}.md via {ENV_INFO} placeholder (correct workspace path and date, no guessing)
  • Final reply returns only a 5-finding summary with a clickable file:// hyperlink — not the full report body
• mod.rs / registry.rs: register DeepResearchAgent as a built-in SubAgent

Core — subagent deadlock fix

• coordinator.rs: set skip_tool_confirmation: true in ExecutionContext for all subagents. Previously false, which caused subagents using permission-requiring tools (e.g. BashTool) to block indefinitely waiting for a confirmation channel that nobody would ever respond to.

Frontend

• utils.ts: add DeepResearch capability profile (analysis: 5, docs: 5, creative: 4, coding: 3, ops: 2)

Test plan

• Build passes: cargo build -p bitfun-core
• Unit tests pass: cargo test -p bitfun-core deep_research
• DeepResearch appears in the subagent list in the Agents UI as a built-in agent
• Triggering DeepResearch via Task tool completes without hanging
• Report is saved to {workspace}/deep-research/ with correct date
• Final reply contains a clickable file:// link (not a code block)
• Recursive self-invocation is not possible (Task tool absent from tool list)