feat(context): inject compact continuity hints without transcript summaries #1789

Feature description

Inject compact continuity hints without transcript summaries.

Codex introduced lightweight history-note hints for fresh context windows. PowerContext can borrow the shape, but not the provider-specific context-window mechanism: provide short, bounded continuity hints derived from durable Handoff/Source evidence, while avoiding full transcript summaries.

Codex prior art to review:

• Add history and notes tools for token-budget sessions openai/codex#39827 - Add history and notes tools for token-budget sessions
• Inject history notes hints into context windows openai/codex#40539 - Inject history notes hints into context windows

Problem and proposed solution

Fresh sessions often need a small orientation hint before they read the full Handoff or evidence. Without a supported hint layer, hosts either inject too much old conversation or rely on informal summaries.

Proposed behavior:

• Define a compact continuity-hint section for PreparedContext or host-facing continuation output.
• Derive hints from committed Handoff, prepared Handoff, and authorized Source evidence.
• Mark hints as untrusted historical orientation, not current instructions.
• Keep a hard byte limit; skip hints when they exceed the budget instead of truncating into misleading text.
• Include enough references to let the agent ask for exact Handoff/evidence if needed.

Potential hints:

• active objective label;
• latest Handoff disposition;
• one-line next action;
• known omissions/blockers;
• selected evidence references.

Alternatives considered

• Summarize the entire transcript. This is expensive, sensitive, and easy to misrepresent.
• Rely only on Memory. Memory is for durable project knowledge, not current work position.
• Always inject full Handoff content. That is safer than transcript summaries, but may still be too large for tight startup contexts.

Additional context

Related PowerContext work:

• PR RFC: Handoff session rollover #1783: rollover reasons and fresh-session Handoff generation.
• tracking(continuity): 宿主历史导入与可评估的上下文连续性 #1574: host session import and context continuity.
• evaluation: validate multi-resolution context projections and progressive retrieval #1426: multi-resolution context projections and progressive retrieval.

Acceptance criteria:

• Hints are explicitly bounded and optional.
• Hints never replace Handoff content or evidence checks.
• Hints are omitted when budget or evidence availability is insufficient.
• The documentation explains that hints are orientation only.