feat(mcp): add scoped chat discovery and mention capsules

[kev1n77]

Summary

• Add an MCP catalog scoped to the active chat mode and workspace.
• Let users select MCP servers from the @ picker, with the entry hidden in Minimal mode.
• Render MCP references as capsules in the composer, sent messages, and restored history.
• Constrain the picker width and truncate long names to prevent horizontal overflow.

Type and Areas

Type: Feature, bug fix, UI/UX

Areas: Rust core, desktop/Tauri, CLI peer host, web UI, remote capability contracts, localization

Motivation / Impact

Users can discover and mention available MCP servers directly from chat. The picker shows server-level entries, and selected references display readable server names while retaining their underlying identifiers.

MCP capsules remain consistent after sending messages or reopening history. Long picker entries no longer stretch the popup across the viewport.

Verification

Checks completed during implementation; not rerun when preparing this PR description:

• cargo test --locked -p openbitfun-core --no-default-features --features mcp-runtime,git --lib agentic::tools::product_runtime:: — 28 passed.
• cargo test --locked -p openbitfun-cli --bin openbitfun peer_host::commands::tests — 2 passed.
• cargo test --locked -p openbitfun-product-domains --no-default-features remote_surface — 19 passed.
• cargo check --locked -p openbitfun-desktop — passed.
• pnpm run check:web — passed.
• pnpm run icons:check — passed.
• pnpm run capabilities:check — passed.
• Focused frontend regression tests covered picker behavior, capsule editing, sent messages, and legacy message restoration.
• git diff --cached --check — passed before commit.

Limitations:

• Desktop registry tests compiled but could not start because of the known Windows/Tauri loader error (0xc0000139).
• The core boundary check was blocked by an existing broken HarmonyOS symlink.
• Full application interaction and live remote scenarios were not exercised.

Reviewer Notes

Catalog policy lives in the shared core and is exposed through thin desktop and CLI adapters.

Peer access uses capability negotiation; older peers receive an explicit unsupported state. Remote workspace discovery is explicitly unsupported.

MCP references retain the existing text representation. Capsule rendering does not introduce a persisted schema change or require migration, and historical text references remain readable.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.