Edit Issue - .issue.db

Title *

Description

## Problem

Activity worker holds a database connection checked out from the pool for the ENTIRE duration of shell command execution. For long-running activities (minutes/hours/days), this severely limits scalability.

## Root Cause

In engine/services/activity_worker.py line 459-475:

```python
with db.get_db_connection() as conn:  # Connection checked out HERE
    ctx = DurableContext(conn=conn, ...)
    result = tool_func(ctx, *args, **kwargs)  # Shell runs for hours
# Connection returned only when shell completes
```

## Impact

- 500 concurrent activities = 500 connections held idle
- Current: 13 processes x 10 max pool = 130 max connections  
- Limits concurrent long-running activities to ~130
- Connections show as idle but still checked out from pool

## Actual Connection Needs

Shell tool only needs connection for:
1. Variable resolution (before shell starts)
2. PID storage (after shell starts, then committed)
3. NOTHING during actual shell execution

## Proposed Fix

Release connection after initial setup:
1. Acquire connection for variable resolution
2. Release connection
3. Execute shell WITHOUT holding connection
4. PID storage uses separate short-lived connection
5. Acquire new connection for completion

## Affected Files

- engine/services/activity_worker.py
- engine/tools/shell_output_streamer.py  
- engine/tools/shell_command.py

Due Date (YYYY-MM-DD)

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Edit Issue #244