fix(llm): tighter refinement — word boundaries, JSON extraction, authoritative sources

Addresses issues found while reviewing the initial phase-2 implementation
against real data:

**Bug: uncertain bucket starved from the LLM.**
`discover_entities` was dropping the regex-uncertain bucket whenever real
git/manifest signal existed — which is exactly when `--llm` is most useful
for cleaning up prose noise. The uncertain candidates never reached the
refinement step. Fixed: only drop when `llm_provider is None`.

**Context collection: word boundaries, not substring.**
`_collect_contexts` used substring matching on lower-cased lines, so the
name "Go" matched "good", "going", "forgot". Switched to a
`(?<!\w)…(?!\w)` regex so short names only match at token boundaries.

**Authoritative-source detection replaces confidence threshold.**
Previously the refinement step skipped entries with `confidence >= 0.95`
to avoid second-guessing manifest-backed projects. That threshold was
fragile — the regex detector produces 0.99 confidence for things like
`code file reference (5x)` on framework names (OpenAPI, etc.), so those
skipped the LLM despite being regex-only noise. New helpers
`_is_authoritative_person` / `_is_authoritative_project` look at the
actual signal strings (commits, package.json, etc.) to decide.

**Now also refines regex-derived people.**
After #1148's high-pronoun-signal fix, the regex detector can promote
non-people to the `people` bucket (e.g. a capitalized common noun that
happened to appear near pronouns). The LLM now gets a chance to clean
those up, while git-authored people are still skipped.

**Robust JSON extraction.**
Small local models routinely wrap JSON output in prose ("Sure, here's
the classification: {…}"). The previous code-fence stripper failed on
that. `_extract_json_candidates` now does balanced-bracket extraction
with string-aware quote handling, so it recovers JSON from:
- raw responses
- markdown fenced blocks
- JSON embedded inside surrounding text
- multiple candidate objects/arrays

**Prompt guidance for frameworks vs user projects.**
Added an explicit instruction: frameworks, runtimes, APIs, cloud
services, and third-party vendors (Angular, OpenAPI, Terraform, Bun,
Google, etc.) are TOPIC unless the context clearly says it's the user's
own codebase. Directly addresses a false-positive pattern observed
during dev runs.

**Defensive mtime.**
`convo_scanner._safe_mtime` catches OSError during `stat()` — permission
changes, filesystem races, broken symlinks — and sorts the affected file
to the end of the newest-first order rather than crashing the scan.

**Cosmetic:** merged two adjacent f-strings on the same line in
`backends/chroma.py` and `llm_client.py` (no behaviour change).

15 new tests cover the OSError fallback, word-boundary matching, JSON
extraction variants, authoritative-source helpers, refining high-
confidence regex projects, and end-to-end LLM refinement preserving the
uncertain bucket.

tests/test_project_scanner.py

@@ -3,6 +3,7 @@
 import json
 import subprocess
 from pathlib import Path
+from types import SimpleNamespace
 
 from mempalace.project_scanner import (
     PersonInfo,
@@ -390,6 +391,49 @@ def test_discover_entities_prefers_real_signal_over_prose(tmp_path):
     assert "realproj" in proj_names
 
 
+def test_discover_entities_keeps_uncertain_for_llm_when_real_signal(tmp_path):
+    """With --llm, regex-uncertain prose candidates should reach refinement."""
+    (tmp_path / "package.json").write_text(json.dumps({"name": "realproj"}))
+    _init_git_repo(tmp_path)
+    (tmp_path / "doc.md").write_text("Noise appeared. Noise repeated. Noise again.")
+
+    class FakeProvider:
+        def __init__(self):
+            self.prompts = []
+
+        def classify(self, _system, user, json_mode=True):
+            self.prompts.append(user)
+            return SimpleNamespace(
+                text='{"classifications": [{"name": "Noise", "label": "COMMON_WORD"}]}'
+            )
+
+    provider = FakeProvider()
+    d = discover_entities(str(tmp_path), llm_provider=provider, show_progress=False)
+
+    assert len(provider.prompts) == 1
+    assert "Noise" in provider.prompts[0]
+    assert "Noise" not in [e["name"] for cat in d.values() for e in cat]
+
+
+def test_discover_entities_keeps_llm_only_project_uncertain_when_real_signal(tmp_path):
+    """Repo roots should not auto-promote LLM-only tools/topics into projects."""
+    (tmp_path / "package.json").write_text(json.dumps({"name": "realproj"}))
+    _init_git_repo(tmp_path)
+    (tmp_path / "doc.md").write_text("Terraform shipped. Terraform changed. Terraform runs.")
+
+    class FakeProvider:
+        def classify(self, _system, _user, json_mode=True):
+            return SimpleNamespace(
+                text='{"classifications": [{"name": "Terraform", "label": "PROJECT"}]}'
+            )
+
+    d = discover_entities(str(tmp_path), llm_provider=FakeProvider(), show_progress=False)
+
+    assert "realproj" in [e["name"] for e in d["projects"]]
+    assert "Terraform" not in [e["name"] for e in d["projects"]]
+    assert "Terraform" in [e["name"] for e in d["uncertain"]]
+
+
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