[codex] Handle rich ACP prompt content

[nick-pape]

Summary

• preserve ACP text, resource link, and image prompt blocks in session logs
• convert prompt content into deterministic fallback text and AI SDK model content
• add configurable ACP image prompt capability and update replay/context handling

Closes #10.

Validation

• node common/scripts/install-run-rush.js build
• node common/scripts/install-run-rush.js test

[Copilot]

Pull request overview

This PR adds first-class handling for rich ACP prompt content (text, resource links, and images) by persisting structured prompt parts in session logs, generating deterministic fallback text renderings, and forwarding image parts to the model when enabled.

Changes:

• Introduces structured prompt/content types in @fledgling/common and updates context building/token estimation to support structured message content.
• Adds ACP prompt conversion utilities in agent-core to persist structured content while producing deterministic fallback text + model-ready content.
• Adds configurable image prompt capability (via env/config) and updates agent replay/session loading to reconstruct rich user history.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
README.md	Documents the new image prompt input env toggle and updated rich prompt limitation notes.
packages/context-builder/src/token-estimator.ts	Estimates tokens from structured message content by projecting it to fallback text.
packages/context-builder/src/context-builder.test.ts	Adds coverage for building replay context from structured user content.
packages/context-builder/src/build-context.ts	Converts stored rich user events into model-ready context content with deterministic fallbacks.
packages/common/src/session-events.ts	Adds structured content types and extends user message events to persist structured prompt content.
packages/agent-core/src/prompt-content.ts	Implements normalization/rendering/model conversion for ACP prompt parts (text/resource_link/image/unsupported).
packages/agent-core/src/prompt-content.test.ts	Adds tests for rich prompt conversion, image forwarding behavior, and unsupported blocks.
packages/agent-core/src/interfaces.ts	Adds optional prompt content capability flags (imageInput) to dependencies.
packages/agent-core/src/agent.ts	Persists structured user content, forwards images when enabled, and advertises prompt image capability.
packages/acp-agent/src/prompt-content.ts	Removes legacy prompt flattening utilities (now handled in agent-core).
packages/acp-agent/src/agent.ts	Enables image prompt input by default (env-toggleable) in the Node agent dependencies.
packages/acp-agent/src/agent-prompt-cancellation.test.ts	Adds tests for prompt capability advertisement, rich prompt persistence, and session reload behavior.
common/reviews/common.public.api.md	Updates public API surface for structured message content types.
common/reviews/agent-core.public.api.md	Updates public API surface for new prompt conversion utilities and options.

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