Move runtime identity into system prompts

[morganmcg1]

Summary

• move the stable advisor/student runtime identity into the rendered role system prompt
• render from an explicit non-secret allowlist, fail on missing placeholders, and persist one supervisor-owned snapshot for worker restarts and delegated children
• remove duplicate user-level identity text, both shell envsubst paths, and the now-unused gettext-base image dependency
• keep mutable launch/operator context at user priority and retain the Add program.md to system context and clean up docs/ #3498 no-overlay prompt architecture

Stacking

This PR is intentionally stacked on #3498 and targets codex/program-md-system-prompt, because the follow-up depends on #3498's program-system-prompt and target-overlay removal. It contains one follow-up commit and does not modify PR #3498.

Validation

• uv run pytest -q tests/test_launch_context.py tests/test_supervisor.py tests/test_controller.py tests/test_delegation.py tests/test_openhands_config.py tests/test_openhands_conversation.py tests/test_image_split.py — 164 passed in 16.13s
• uv run pytest -q — 930 passed, 6 skipped in 99.00s
• bash -n k8s/*.sh scripts/*.sh plugins/senpai/scripts/*.sh — passed
• uv lock --check — passed (402 packages resolved)
• git diff --check — passed