Give extensions the browser and clipboard back, and correct the README

[Seungpyo1007]

First item on track C of the 0.3.0 plan. It started as a README correction and turned out to be two one-line fixes plus a README correction.

What the README claimed

Each of those throws rather than returning an empty value, so an extension that needs one fails loudly instead of quietly doing nothing.

Two of the three were right:

Gap	Actual behaviour
WorkspaceEdit create/delete/rename	throws (vscodeShim.ts:146-148) ✓
Custom editors, debug adapter API	absent from the API object → TypeError ✓
TextEditor decorations	silent no-op ✗

Two more the README never mentioned

openExternal: () => Promise.resolve(true),                     // opens nothing, reports success
clipboard: { writeText: () => Promise.resolve(),               // writes nowhere
             readText: () => Promise.resolve("") },            // always empty

openExternal reported success without calling schutz:openExternal — an IPC that has existed all along, complete with scheme validation. The clipboard stubs sat next to six working navigator.clipboard call sites in the app itself.

From an extension's side this reads as its own bug: it writes to the clipboard, reads it back, and finds it empty.

This change

• env.openExternal goes through the real IPC. The main process only opens http/https/mailto, and a refusal is passed back rather than reported as success.
• env.clipboard goes through navigator.clipboard.
• Decorations need an implementation, so the README now names them as the one place left that answers successfully and draws nothing — better than asserting the opposite. Building it is the next item on this track.

Verification

npm run typecheck clean, 981 unit tests passing, npm run build succeeds.

No real-app probe on these two: driving them would open a browser window and overwrite the clipboard on the machine running the test. The wiring was checked by reading — the IPC exists and validates schemes, and the clipboard path is the one the app already uses. Running an extension through them belongs with the decoration work.