feat(selfhost): make global .loopover.yml a real base layer; migrate duplicateWinnerMode off env var

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Summary

Part of #6275 (Phase 1, the proof-of-pattern case). Two changes, same PR:

1. Make the global .loopover.yml (config-dir root) a genuine base layer: today's loader
   (src/selfhost/private-config.ts) treats it as a fallback candidate only, tried after every
   per-repo candidate path and used only when NONE of them exist. Since every currently-configured
   repo already has its own per-repo file, the global file is never actually consulted for any of
   them. It needs to become a real base layer that a per-repo inherit value defers to.
2. Migrate duplicateWinnerMode's "inherit" resolution off LOOPOVER_DUPLICATE_WINNER
   (src/settings/duplicate-winner-mode.ts) so it reads the global .loopover.yml's own
   settings.duplicateWinnerMode value first, falling back to the env var only when the global yml
   doesn't specify it either.

Why

PRs #6248/#6249 both linked the same issue; #6248 (created first) should have survived as the
duplicate-cluster winner (#dup-winner) but was auto-closed identically to #6249, because the
winner-adjudication feature was off fleet-wide — controllable only via an env var in .env, with no
way to turn it on from .loopover.yml at all (global or per-repo inherit).

Requirements

• No behavior change for any existing repo config unless the global .loopover.yml is explicitly
  edited to add settings.duplicateWinnerMode.
• A per-repo duplicateWinnerMode: off or duplicateWinnerMode: enabled override still wins over
  whatever the global yml says (per-repo always outranks global, matching every other config-as-code
  field's precedence in this codebase).
• LOOPOVER_DUPLICATE_WINNER keeps working as the final fallback when NEITHER the global yml nor the
  per-repo yml specifies a value — this is additive, not a breaking change to the env var.

[linear-code]

JSO-4365

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Correction: this sub-issue's premise was wrong. The global .loopover.yml is not fallback-only —
src/selfhost/private-config.ts already deep-merges shared-base → global → per-repo layers (#1959 /
PR #3660, live in production since long before this issue was filed). Traced
resolveDuplicateWinnerEnabled directly: when the merged manifest's settings.duplicateWinnerMode is
"enabled" (not "inherit"), it returns true immediately without ever consulting the env var — so
setting it in the global yml already worked, zero code change needed.

Applied directly: settings.duplicateWinnerMode: enabled added to the global .loopover.yml on the
self-host instance, and the now-redundant LOOPOVER_DUPLICATE_WINNER=true removed from .env.
Verified: config parses clean, app healthy, no manifest warnings in logs.

Closing as resolved by config change, not code.