Add shell completion for update_status.py

[baramgay]

Summary

Add tab-completion support for update_status.py so users can tab-complete agent IDs and status values in bash/zsh/fish.

Motivation

The CLI is used dozens of times per session. Tab-completion would reduce typos and speed up the workflow.

Expected behavior

python scripts/update_status.py [TAB]
orchestrator  lead-data  lead-dev  eda-analyst  backend ...

Acceptance criteria

• Completes agent IDs from agent_status.json
• Completes status values (working/review/done/idle)
• Works in bash, zsh, fish
• Installation instructions added to README

This is a good first issue — mostly argument parsing + argcomplete library.

[baramgay]

Implemented in PR #6.

What's in the PR:

• argcomplete integration for update_status.py: tab-completes agent IDs and status values
• New scripts/install_completion.py: one-command setup for bash, zsh, or fish

Usage:
pip install argcomplete
python scripts/install_completion.py --bash # or --zsh / --fish
source ~/.bashrc
python scripts/update_status.py [TAB]

orchestrator lead-data lead-dev eda-analyst backend ...

No changes needed to your existing workflow — argcomplete is optional and fails gracefully if not installed.