fix(local): route multi-signature packages to human triage in the proposer (#119)

## Problem

A live `propose-patches.R --open-issue` run aborted with:

```
Patch registry validation FAILED:
  - ambiguous duplicate entries: hmmTMB|ubuntu-2604|*, imt|ubuntu-2604|*, refundBayes|ubuntu-2604|*
Error: Candidate registry failed validation; not writing or proposing.
```

Root cause: a package whose failing builds match **more than one** auto-proposable signature (e.g. some logs hit `tbb-stddef-removed`, others `rcppparallel-bundled-tbb`) landed in two separate signature groups, so the proposer emitted two candidate entries with the same `package|platforms|versions` key. The validator's ambiguous-duplicate check then failed the whole candidate set, aborting the run — so even the unambiguous candidates (e.g. `vacalibration`) never got proposed.

## Fix

Add a pure, tested `dedupe_candidates()` that:

- keeps a package's candidate only when it maps to a **single** signature (collapsing an accidental repeat under the same signature), and
- routes a package that maps to **multiple** signatures (conflicting fix tiers -- makevars vs source patch) to **human triage** instead of guessing between them.

The proposer prints the skipped ambiguous packages and proceeds with the clean candidates, so one ambiguous package no longer blocks the rest. This matches the issue #115 guardrail: don't auto-pick when the fix is ambiguous.

## Verification

- New unit tests in `test-proposal-tracking-lib.R` cover the single-signature, multi-signature (conflict), duplicate-under-one-signature, and empty cases.
- Reproduced the original failure end-to-end with a stubbed DB (`hmmTMB` ambiguous + `vacalibration` clean): `hmmTMB` is now listed under "Ambiguous -> human triage", `vacalibration` is proposed, and the candidate registry validates (`Patch registry OK`).
- Full suite: 78 tests pass; all pre-commit hooks pass.

Reviewed-on: #119

local/propose-patches.R

@@ -135,9 +135,25 @@ for (r in report) {
   }
 }
 
+# A package that maps to more than one auto-proposable signature is ambiguous
+# (conflicting fix tiers) and would collide on the same registry key; route it
+# to human triage instead of emitting both.
+split_candidates <- dedupe_candidates(candidates)
+candidates <- split_candidates$keep
+if (length(split_candidates$ambiguous) > 0L) {
+  cat("\nAmbiguous (multiple signatures) -> human triage, not proposed:\n")
+  for (pkg in names(split_candidates$ambiguous)) {
+    cat(sprintf(
+      "  %s: %s\n",
+      pkg,
+      toString(split_candidates$ambiguous[[pkg]])
+    ))
+  }
+}
+
 if (length(candidates) == 0L) {
   cat(
-    "No auto-proposable candidates (nothing classified, safe, and unregistered).\n"
+    "\nNo auto-proposable candidates (nothing classified, safe, unregistered, and unambiguous).\n"
   )
   q(status = 0)
 }