improve template arg to accept remote and local json files

[bmacer]

📥 Pull Request

📘 Description
Addresses #90

• Accept a local .json file as a template
• Accept a remote https file as a template
• Check for existence of these files, with decent error messages
• Improve error messaging when using built-in template

🧪 Testing

Remote file doesn't exist:

agentstack init --template=https://raw.githubusercontent.com/AgentOps-AI/AgentStack/refs/heads/main/agentstack/templates/proj_templates/research2.json
Failed to fetch template data from https://raw.githubusercontent.com/AgentOps-AI/AgentStack/refs/heads/main/agentstack/templates/proj_templates/research2.json

Local file doesn't exist:

agentstack init --template=local-template-file-doesnt-exist.json
Template file does not exist: /Users/bmacer/Desktop/home/agentstack/newAgentStack/AgentStack/local-template-file-doesnt-exist.json

Remote file does exist:

agentstack init --template=https://raw.githubusercontent.com/AgentOps-AI/AgentStack/refs/heads/main/agentstack/templates/proj_templates/research.json

🚀 AgentStack project generated successfully!

  Next, run:
    cd research
    python -m venv .venv
    source .venv/bin/activate
    poetry install

  Add agents and tasks with:
    `agentstack generate agent/task <name>`

  Run your agent with:
    agentstack run

  Run `agentstack quickstart` or `agentstack docs` for next steps.

🔨 Tool perplexity added to agentstack project successfully

Local file does exist:

agentstack init --template=system_analyzer.json

🚀 AgentStack project generated successfully!

  Next, run:
    cd system_analyzer
    python -m venv .venv
    source .venv/bin/activate
    poetry install

  Add agents and tasks with:
    `agentstack generate agent/task <name>`

  Run your agent with:
    agentstack run

  Run `agentstack quickstart` or `agentstack docs` for next steps.

🔨 Tool directory_search added to agentstack project successfully
🔨 Tool file_read added to agentstack project successfully
🔨 Tool code_interpreter added to agentstack project successfully

Built-in template does exist:

python -m agentstack.main init --template=content_creator

🚀 AgentStack project generated successfully!

  Next, run:
    cd content_creator
    python -m venv .venv
    source .venv/bin/activate
    poetry install

  Add agents and tasks with:
    `agentstack generate agent/task <name>`

  Run your agent with:
    agentstack run

  Run `agentstack quickstart` or `agentstack docs` for next steps.

🔨 Tool perplexity added to agentstack project successfully

Built-in template doesn't exist:

agentstack init --template=template-doesnt-exist
No known built-in template: template-doesnt-exist
Available built-in templates:
content_creator
research
system_analyzer