fix(context): make ContextGraph.add_edge idempotent by deduping on edge_id - #926

edge_id is content-derived and not yet guaranteed unique (semantica-agi#922, fix
pending in semantica-agi#926): two identical add_edge() calls produce two edge
objects sharing one id. retract_edge()/purge_edge() resolved "the
edge" via the first matching object only, so a duplicate was silently
left untouched (still live, still active) while the call returned
True and recorded a tombstone/retraction claiming it was fully
handled. Repeat purge_edge() calls also silently overwrote the
tombstone's reason/purged_at on each partial attempt instead of
no-op'ing once nothing remained to purge.

retract_node()'s cascade had the same root cause from the other
direction: it checked the live _retractions dict mid-loop, so the
first duplicate's just-written record made the second look already
handled and it was skipped outright, left permanently active.

retract_edge()/purge_edge() now act on every edge matching the id
under a single record; the cascade's dedup check is snapshotted
before the loop starts so within-call duplicates are still closed
rather than skipped.

Adds TestDuplicateEdgeId (5 tests) reproducing all three paths.

* feat(context): add retraction and purge to ContextGraph

ContextGraph had 56 public methods and none that removed anything: the only
option was clear(), which discards the whole graph. Removing one entity meant
exporting to a dict, filtering by hand and rebuilding, losing provenance.

Add two operations with deliberately different contracts.

retract_node/retract_edge close the entity's validity window. The entity stops
being active going forward, but state_at() before the retraction still returns
it, so decisions recorded against it remain explainable. This reuses the
valid_from/valid_until machinery already present rather than adding a new
subsystem.

purge_node/purge_edge remove the entity outright, from history as well as from
the active view, leaving a tombstone that records that a purge happened and why
but never the purged content. Scope is this graph only; copies in AgentMemory
or a bound vector store are not reached, so it is one step of an erasure
workflow rather than the whole of it.

Both record themselves through the existing mutation_callback path.
MutationRecord already documented REMOVE_NODE/REMOVE_EDGE in its operation
vocabulary, so retraction emits UPDATE_NODE and purge emits REMOVE_NODE with no
changes required to change_management.

Incident-edge lookup scans self.edges rather than _adjacency, which is keyed by
source only and would otherwise leave inbound edges pointing at a removed node.
Purge updates edges, edge_type_index and _adjacency together so the indexes
cannot drift, and clear() now resets the retraction and tombstone records.

* fix(context): address review findings on retraction and purge

* fix(context): close every duplicate when retracting/purging by edge_id

edge_id is content-derived and not yet guaranteed unique (#922, fix
pending in #926): two identical add_edge() calls produce two edge
objects sharing one id. retract_edge()/purge_edge() resolved "the
edge" via the first matching object only, so a duplicate was silently
left untouched (still live, still active) while the call returned
True and recorded a tombstone/retraction claiming it was fully
handled. Repeat purge_edge() calls also silently overwrote the
tombstone's reason/purged_at on each partial attempt instead of
no-op'ing once nothing remained to purge.

retract_node()'s cascade had the same root cause from the other
direction: it checked the live _retractions dict mid-loop, so the
first duplicate's just-written record made the second look already
handled and it was skipped outright, left permanently active.

retract_edge()/purge_edge() now act on every edge matching the id
under a single record; the cascade's dedup check is snapshotted
before the loop starts so within-call duplicates are still closed
rather than skipped.

Adds TestDuplicateEdgeId (5 tests) reproducing all three paths.

* docs(changelog): document retraction/purge feature

Adds an Unreleased/Added entry for #955/#957 covering retract_node,
retract_edge, purge_node, purge_edge and the get/list accessors, plus
the duplicate-edge_id fix caught and applied during review.

---------

Co-authored-by: Pravit Ampapathini <pravitampapathini@Pravits-MacBook-Air-3.local>
Co-authored-by: KaifAhmad1 <kaifahmad087@gmail.com>