feat(semantic-spacetime): add M4 cross-links and final validation fixes

Add the M4 integration cross-links: a single reverse link in
promise-theory/references/foundations.md section 3.12 pointing to the
semantic-spacetime skill (the only promise-theory change), and one
FAILURE-MODE-INDEX.md row for semantic drift / meaning disagreement
following the file's concrete-trigger + real-skill-path rule. Refresh
stale milestone forward-references in semantic-spacetime/references/
foundations.md now that the M2 references and M3 tooling exist. Full
validate.yml-equivalent suite passes locally: 152 canonical skills, 96
eval manifests schema-valid, eval-coverage ratchet green (63.2%),
check-artifacts + jscpd + ruff/mypy/radon/deptry/bandit green, and the
fake-adapter eval smoke runs 6 trials with 0 failures.

Co-authored-by: factory-droid[bot] <138933559+factory-droid[bot]@users.noreply.github.com>

FAILURE-MODE-INDEX.md

@@ -10,5 +10,6 @@ Use this index when the work is blocked by a recognizable failure pattern rather
 | A multi-agent decision is converging too early | One perspective appears sufficient but alternatives have not been tested | [agent-council](agent-council/SKILL.md) |
 | A contribution may duplicate existing work | The requested feature sounds like something already in the repository | [opensource-contributions](opensource-contributions/SKILL.md) |
 | A workflow is repeated manually | The same sequence of skills is executed more than once | [workflow-architect](bundles/workflow-architect/SKILL.md) |
+| Meaning drifts or disagrees between agents | Agents use the same word for different meanings, or semantic divergence and dead-ends block progress | [semantic-spacetime](semantic-spacetime/SKILL.md) |
 
 This is a routing aid, not a replacement for reading the selected skill. Add a row only when the failure mode has a concrete trigger and an existing skill that addresses it.

promise-theory/references/foundations.md

@@ -391,7 +391,9 @@ Promise graphs are directed graphs whose edges are promises; since each edge
 requires a counter-promise to be *effective*, "a link requires the mutual
 consent of two autonomous agents," making promise graphs more primitive than
 ordinary graph adjacency — and the foundation for Burgess's notion of *semantic
-spacetime* (Burgess, "Spacetimes with Semantics (I)," arXiv:1411.5563, 2014).
+spacetime* (Burgess, "Spacetimes with Semantics (I)," arXiv:1411.5563, 2014);
+the skill that develops this downstream concept is
+[semantic-spacetime](../../semantic-spacetime/SKILL.md).
 Graph-inspection is where broken promises show up: two promises of the same
 type with different constraints are a contradiction (Burgess, "Promise You A
 Rose Garden," 2007).

semantic-spacetime/references/foundations.md

@@ -9,7 +9,8 @@ time, causality, the promise-theory substrate, and adjacent fields. What does
 not belong here: quantum-gravity or physics derivation (this is not a physics
 theory — see §4), the CFEngine/infrastructure application history, and the
 agent-coordination synthesis; those belong to the skill's application and
-agent-coordination references, which are added in a later milestone. For
+agent-coordination references ([applications-infrastructure.md](applications-infrastructure.md)
+and [agent-coordination.md](agent-coordination.md)). For
 one-line definitions see [glossary.md](glossary.md); for sources see
 [bibliography.md](bibliography.md).
 
@@ -188,7 +189,7 @@ injection of remedial information"* — and the boundary where the graph leaks i
 outgoing LEADS TO/EXPRESSES edges that matter) accumulates meaning and stops
 propagating it; intent or policy must be injected manually at that boundary.
 For a bounded diagnosis procedure using this concept, see the skill's
-diagnosis-and-debugging reference (added in a later milestone).
+[diagnosis-and-debugging.md](diagnosis-and-debugging.md) reference.
 
 ## 4. The "not physics" boundary
 
@@ -285,8 +286,9 @@ requires measurement** — meaning cannot be asserted before the dynamics are
 measured at the right scale. Different scales yield different conclusions; a
 measurement that is stable at one scale can be wrong at another. This duality
 is the theory-level ground for the "dynamics always trumps semantics" lesson of
-the infrastructure lineage (covered in the application reference, added in a
-later milestone) and for Gotcha 4 in SKILL.md.
+the infrastructure lineage (covered in the application reference,
+[applications-infrastructure.md](applications-infrastructure.md)) and for
+Gotcha 4 in SKILL.md.
 
 ## 8. Distance: metric vs semantic
 
@@ -302,7 +304,7 @@ Part III defines two kinds of distance [VERIFIED — arXiv:1608.02193v4]:
 The distinction is operational: two concepts can be close in coordinates yet
 far in interpretation, and vice versa. A weighted hop count over a γ(3,4) graph
 is a semantic-distance instance of the hop-count family — the family this
-skill's model tooling will implement for measuring drift between two snapshots
+skill's model tooling implements for measuring drift between two snapshots
 of a system's meaning.
 
 ## 9. Learning and knowledge