docs: drop the Orchestrate principle and scope the upstream rule

[Tryanks]

Changes

• Removes principle 6 (Orchestrate is core). The WASM plugin roadmap is a long-term plan, not a development constraint. Principles 7–9 become 6–8, and the one reference by number ("Principle 9 governs anything beyond this repository") now points to 8.
• Narrows the upstream principle (now 8). A problem caused by Tcode (its code, its tests, or how it uses a dependency) is fixed where it is found, as part of the current work. Only a problem caused by the upstream itself is carried and reported. The rest of the principle is unchanged.
• Localize step in "Problems that may be a dependency's". It now says that when the cause is Tcode's, you fix it and stop; the later steps apply only when the cause is in the dependency.

Checks

This changes documentation only, with no code or tests. CI runs the full suite on this push.