fix(agent): word-boundary matching + contracted negatives in chat-assent

Fix A1 of plans/2026-07-11-nomos-agent-code-review.md. isAssent and
isTypedConfirmation used a space-padded word-boundary check for negation
words but a bare strings.Contains for assent/confirm words — confirmed live
via test probes: isAssent("...maybe yesterday's logs...") returned true
("yes" matched inside "yesterday"), and isTypedConfirmation("I haven't
confirmed anything yet") returned true ("confirm" matched inside "confirmed",
and "haven't" wasn't in negationWords — only "don't"/"do not" were).
isTypedConfirmation is the sole gate for DESTRUCTIVE actions, so the second
case meant a message merely stating something hadn't been confirmed could
read as an explicit confirmation.

- Replaced the ad-hoc space-padding/prefix-check negation logic with proper
  tokenization (wordTokenRe) + containsPhrase, matching WHOLE tokens/phrases
  only — never a mid-word substring. Handles curly apostrophes too (a
  pre-existing gap: the old straight-quote-only check would have missed
  "don't" typed with a smart quote).
- Added contracted negatives (haven't, hasn't, isn't, wasn't, aren't, can't,
  cannot, won't, wouldn't, shouldn't, didn't, doesn't) to negationWords.
  Deliberately did NOT add a bare "not" — too broad, would false-negative
  ordinary assent like "go ahead, this is not risky".
- Added regression tests for both confirmed cases plus a couple of adjacent
  ones (eyesight/isn't, can't confirm) so a future change can't silently
  reintroduce either bug.

All existing assent/confirmation tests pass unchanged — this is a pure
robustness fix, not a behavior change for any previously-correct case.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

cmd/nomos/assent_test.go

@@ -45,6 +45,43 @@ func TestIsAssent_NegationBeatsAssentWord(t *testing.T) {
 	}
 }
 
+// TestIsAssent_WholeWordBoundary regression-tests a real false positive found
+// live: the old substring check matched "yes" inside "yesterday" (and would
+// equally match "confirm" inside "confirmed"/"unconfirmed" for
+// isTypedConfirmation below) because only negation used a word-boundary
+// check — assent/confirm words used a bare strings.Contains. Confirmed via a
+// throwaway probe before being fixed; kept here permanently so a future
+// change can't silently reintroduce it.
+func TestIsAssent_WholeWordBoundary(t *testing.T) {
+	cases := []string{
+		"not sure, maybe yesterday's logs show something useful",
+		"my eyesight isn't great, what does that say",
+	}
+	for _, c := range cases {
+		if isAssent(c) {
+			t.Errorf("isAssent(%q) = true, want false (word-boundary: 'yes' must not match inside 'yesterday'/'eyesight')", c)
+		}
+	}
+}
+
+// TestIsTypedConfirmation_ContractedNegation regression-tests the other real
+// false positive: isTypedConfirmation gates DESTRUCTIVE actions, and
+// "confirm" matching inside "confirmed" combined with contracted negatives
+// ("haven't") not being in negationWords meant a message that explicitly
+// says the operator has NOT confirmed something could read as confirming it.
+func TestIsTypedConfirmation_ContractedNegation(t *testing.T) {
+	cases := []string{
+		"I haven't confirmed anything yet, let me think",
+		"that isn't confirmed on my end",
+		"we can't confirm that until tomorrow",
+	}
+	for _, c := range cases {
+		if isTypedConfirmation(c) {
+			t.Errorf("isTypedConfirmation(%q) = true, want false (contracted negation should block)", c)
+		}
+	}
+}
+
 func TestIsTypedConfirmation(t *testing.T) {
 	positive := []string{
 		"I confirm destroy 135 in strong",