Allow duplicate display names with unambiguous resolution

[schickling-assistant]

Why

Display names are mutable presentation metadata, so requiring global uniqueness makes harmless relabeling an identity operation. Stable session IDs already provide the durable identity needed by automation.

Part of compoundingtech/st2#128.

What

• allow duplicate display names and display-name/stable-ID collisions while preserving stable-ID uniqueness and label validation
• resolve an exact stable ID first, a unique display name second, and fail closed with candidate IDs when a display name is ambiguous
• apply the same resolver to every local CLI ref consumer and fabric remote routing
• document the API, CLI, and on-disk contract

Validation

• npm run typecheck - pass
• npm run build - pass
• CI=1 ./node_modules/.bin/vitest run tests/display-name.test.ts tests/remote-fabric.test.ts tests/up-name-decouple.test.ts - 59/59 pass
• CI=1 npm test - 1,471 pass, 20 skip, 4 unrelated environment failures: zsh is absent; Vim resolves to Neovim 0.12.3 and Nano 9.0 renders different text than the screenshot expectations
• npm run verify-docs - 12/13 pass; the Vim example hits the same Neovim text mismatch