fix: use template pattern for credential injection and improve docs

[ps48]

Description

• Update installer script coloring from this PR: Refresh installer CLI UI: purple theme, cleaner layout, better CX #41
• Data Prepper's pipelines.yaml hardcoded credentials in 3 OpenSearch sink sections
• The installer used fragile sed replacements to patch them at install time, which failed silently with custom passwords Data Prepper gets Unauthorized access
• The dashboards init script had a wrong default password fallback ("admin" instead of "My_password_123!@#")
• AGENTS.md had incorrect password values in 4 places
• README lacked port reference, installer flag docs, and credential workflow details

Changes

Credential template pattern (matches OpenSearch OTel Demo approach):

• Renamed pipelines.yaml → pipelines.template.yaml with OPENSEARCH_USER/OPENSEARCH_PASSWORD placeholders
• Added command: to data-prepper service in docker-compose.yml that runs sed at container startup to inject credentials from .env
• Removed 2 fragile sed lines from install.sh .env is now the single source of truth

Bug fixes:

• Fixed default password fallback in init-opensearch-dashboards.py ("admin" → "My_password_123!@#")
• Fixed 4 incorrect password values in AGENTS.md

Documentation improvements (README.md):

• Added Ports reference table for all core services, example services, and OTel Demo endpoints
• Added Installer flags table (--simulate, --skip-pull, --help) with usage examples
• Expanded credential section: default creds, setting during install, changing after install, and how the template pattern works

Doc updates across all files:

• Updated config file paths from pipelines.yaml → pipelines.template.yaml in CLAUDE.md, AGENTS.md, README.md, docker-compose/README.md
• Simplified credential change instructions everywhere no more manual sed

Fixes: #43
Resolves: #38

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