Copy thread references cannot be opened outside T3 Code #28151

Problem

The existing Copy > Thread action copies an internal Markdown/context reference. It can be used inside T3 Code, but pasting it into another application does not provide a normal URL that opens the referenced thread. Users need one copied value that works both in external applications and as a thread card in the T3 Code composer.

Reproduction

Baseline: upstream main at 79f1fad, verified in an isolated Windows web client.

1. Open a thread's sidebar context menu and choose Copy > Thread.
2. Paste into a plain-text destination outside T3 Code.
3. The value is an internal reference rather than a public address that the destination can open.

Requested behavior and scope

• Keep the existing Copy > Thread action and copy a plain public URL identifying the owning environment and thread.
• Open supported thread links through the existing web, desktop, and mobile navigation and connection flows. Opening a link does not grant access to its environment.
• Pasting a supported URL into the composer creates the existing thread context card; keep legacy context references working.
• Preserve inaccessible-environment URLs as text instead of creating unusable cards.

Implementation and before/after browser evidence: #28148. Native iOS/Android paste execution and OS protocol activation remain unverified.

Maintainer decision

This issue records the problem and requested scope. It is not evidence of prior maintainer approval. Elliot has stated that approval was obtained, but its source URL has not been supplied. Please confirm the direction and scope publicly, or link the existing approval, so the PR can cite that record under CONTRIBUTING.md.