[Prompt registry] Phase 4 — Feature payoffs: shell-aware prompt (#85) + interaction variant (#1317)

Part of the prompt-registry roadmap OpenHands/software-agent-sdk#3606 (proposal OpenHands/software-agent-sdk#2827).

Goal: ship the two feature payoffs that justify the refactor. Both can land in parallel once the registry is the default path.

Depends on: Phase 3 OpenHands/software-agent-sdk#3611.
Relates to: revives #85 and OpenHands/software-agent-sdk#1317 (both closed NOT_PLANNED; approach is now section-based).

Tasks

• ShellGuidanceSection + delete refine() (revives #85). Section describes the actually-bound shell tool from ctx.tool_names / ctx.platform, so the prose can no longer drift. Remove refine() (context/prompts/prompt.py:48), its render-time call at :114, and the Jinja filter registration at :73. Add posix-vs-windows section tests.
• InteractionVariant (revives OpenHands/software-agent-sdk#1317**).** EfficiencySection + VerificationPolicySection + FinalMessageFormatSection, gated by ctx.interaction_mode; wire cli_mode / interaction mode from the presets (openhands-tools/openhands/tools/preset/*.py) into PromptContext. Salvage wording from the orphaned system_prompt_interactive.j2 / system_prompt_long_horizon.j2 before they are deleted in Phase 5. Snapshot-diff interactive vs non-interactive; non-interactive output stays unchanged.

Acceptance criteria

• Windows agent prompt references execute_powershell / PowerShell with no refine() involved; #85 goal met.
• Interactive mode produces a measurably terser, less-verification-heavy prompt; non-interactive snapshot unchanged; OpenHands/software-agent-sdk#1317 goal met.