Fix Activity History idle, pagination, and Input/Output UX - #55

Summary

• stabilize historical/current Idle calculation and realtime MCP cache behavior
• add true Idle-only pagination plus numeric page/page-size controls
• add Input / Output display controls with full argv, redacted explicit env overrides, and full streamed process output
• recover orphaned MCP request spans with request heartbeats while preserving workspace-wide multi-client Idle semantics

Notable implementation details

• pinned Running rows remain pinned, but no longer widen the historical MCP span range
• Idle remains computed only and is never persisted
• process output is streamed in bounded 32 KiB chunks for Activity History while process_output keeps its existing bounded LLM buffer
• environment secrets are redacted before persistence; inherited process environment is never recorded
• stale MCP requests end at their last known heartbeat so cleanup does not hide real Idle time

Validation

• go test ./coderd/mcp -run 'TestActivity|TestPersistent|TestPersist' -count=1
• go test ./coderd -run 'TestParseWorkspaceCommandActivityQuery|TestNormalizedWorkspaceCommandActivitySource|TestWorkspaceCommandActivityFilterValidation' -count=1
• go test ./coderd/agentapi -run '^TestCommandActivity$' -count=1
• go test ./agent -run '^TestCommandActivityOutputWriter$' -count=1
• relevant go test ./agent/agentproc ...
• go test ./coderd/workspaceactivityredact -count=1
• compile-only checks for ./coderd/database and ./coderd/database/dbauthz
• pnpm run lint:types
• Activity History Vitest suite: 5 files / 27 tests passed
• Biome: 11 files clean
• git diff --check

make pre-commit reaches repository golden generation but cannot complete in this workspace because DB-backed golden tests require /var/run/docker.sock; failures are exclusively dbtestutil container startup errors. The relevant targeted checks above pass.

Closes #54