[feat] Add site-aware content adaptation actions

[ThreeLight2000]

Motivation

Context OS already detects the active browser URL and uses it when saving records. This feature makes the current site an execution context so the extension can expose platform-specific content actions without silently replacing the user draft.

The initial use case is adapting a Korean memo for X or Threads before the user reviews and publishes it.

Scope

In scope:

• Resolve supported site capabilities from the active URL hostname.
• Show Adapt for X on x.com and www.x.com.
• Translate Korean drafts into natural English for X and respect a configurable post limit, defaulting to 280 characters.
• Show Adapt for Threads on threads.net and www.threads.net.
• Preserve Korean and rewrite Threads drafts in a casual, monologue-style voice with a configurable post limit, defaulting to 500 characters.
• Split long content into ordered posts at sentence or paragraph boundaries when possible.
• Render actions through a generic SiteCapability and ContextAction model, separate from the UI.
• Show the generated content in a review preview with explicit Apply and Copy controls.
• Keep the original Memo unchanged until the user explicitly applies the generated content.
• Persist per-platform post limits with the existing extension settings and provide defaults for new or migrated data.

Out of scope:

• Automatically publishing to X or Threads.
• Automatically saving generated content as a separate Context record.
• Media-aware or account-plan-aware real-time platform limit detection.
• GitHub, LinkedIn, Gmail, or other future platform adapters beyond the initial capability model.

Acceptance criteria

• The active hostname resolves to zero or more site capabilities through a generic resolver.
• X and Threads capabilities are implemented for their supported hosts.
• The contextual action is hidden on unsupported sites.
• The action is disabled for an empty or whitespace-only Memo.
• Disabled actions still expose accessible hover and keyboard-focus usage guidance.
• X output is natural English, preserves the draft intent, and each ordered post respects the configured limit.
• Threads output remains Korean, uses a casual monologue style, and each ordered post respects the configured limit.
• Long output is split into ordered posts without losing content or thread boundaries.
• Generated output is shown separately for review and does not mutate the original Memo automatically.
• Apply is an explicit replacement of Memo and Copy preserves the ordered post separation.
• Platform rules and splitting logic are owned by platform adapters or actions, not by the UI component.
• Brain Server exposes a generic adapt-content action with validated input and ordered posts output.

Verification plan

• Add unit tests for hostname matching, supported aliases, and unsupported pages.
• Add client tests for action visibility, empty Memo disabled state, request errors, preview behavior, and original draft preservation.
• Add Brain Server tests for action registration, input and output validation, X and Threads prompts, configurable limits, and long-content splitting.
• Add storage migration tests for platform limit defaults.
• Run the Chrome client typecheck, tests, build, and verify scripts.
• Manually verify X, Threads, unsupported sites, empty Memo, short content, long content, Apply, and Copy flows.

Implementation notes

Use the existing Brain Server Action Registry and /v1/actions endpoint. Introduce a generic SiteCapability / ContextAction contract on the client and a generic adapt-content action on Brain Server with platform-specific adapters.

The repository Project is Context OS Work. The Priority field exists but currently has no configured options, so Priority must not be set to an invented value; configure an official option before assigning it.

[ThreeLight2000]

Implemented on branch feat/28-site-aware-content-adaptation.

Highlights:

• Added generic client SiteCapability / ContextAction resolution for x.com, www.x.com, threads.net, and www.threads.net.
• Added Brain Server adapt-content action with separate X and Threads adapters, platform-aware length measurement, and ordered long-content splitting.
• Added configurable per-platform limits with migration-safe defaults: X 280, Threads 500.
• Added contextual actions with disabled empty-Memo guidance and a separate preview with explicit Apply and Copy controls.
• Original Memo is preserved until Apply is explicitly selected.

Verification passed:

• pnpm --filter context-shelf verify
• pnpm --filter server-brain verify
• pnpm typecheck
• pnpm test
• pnpm build
• pnpm test:repo

The Context OS Work Project item is In progress. Priority was not set because the configured Priority field currently has no options.