feat: raw-text observe, identity provenance, knowledge-graph service

[rrader26]

Go parity with @memmesh/sdk v0.9.0 — see
thinkfleet-memory-sdk#21.

Go was the furthest behind of the five SDKs: it had no raw-text Observe at
all, only the legacy verbatim path posting to /admin/memory.

Observe reaches the engine

res, _ := c.Memory.Observe(ctx, memmesh.Observe{
    Text:      "I just moved to Denver.",
    UserID:    "user-123",   // provenance on whatever the engine keeps
    SessionID: "thread-456", // keeps a conversation's turns linkable
})
// res.Saved, res.CandidateCount

Set Text and the raw turn goes to /memory/observe — extract → dedupe → graph
→ embed — and comes back with what survived. CandidateCount is what extraction
proposed; Saved is what was kept. Filler returns an empty Saved, which is
the engine working, not an error.

UserID / AgentID / SessionID are forwarded, omitted when unset. Provenance,
not a tenancy boundary — search filters chatIdentityId IS NULL OR = $1,
permissive by design.

The legacy Content path still posts verbatim to /admin/memory, wrapped in the
same response shape so callers don't branch on which path ran.

New: c.Graph

Method	Endpoint
Stats(ctx)	GET /admin/memory/graph/stats
ListEntities(ctx, params)	GET /admin/memory/entities
GetEntity(ctx, id, asOf)	GET /admin/memory/entities/:id
ListEdges(ctx, asOf, limit)	GET /admin/memory/graph/edges
Traverse(ctx, id, params)	POST /admin/memory/graph/traverse

Use Stats, not len(ListEntities(..)) for any "how big is it" question.
The list routes page, so their length is the page size — against a real project,
1,000 vs the actual 12,142.

Edges come back hydrated

ListEdges, Traverse, and GetEntity().Edges return GraphTraversalEdge:
Subject and Object are full entities, not ids, plus a Hop counter. The
raw memory_edge row is deliberately not modelled — no read route returns it,
and assuming otherwise broke the Rust port outright with
missing field 'subjectId'.

Hop is 0 from ListEdges (no seed) and 1-indexed from Traverse.

Breaking

Observe returns *ObserveResponse, was *MemoryItem. On the legacy Content
path the old value is now res.Saved[0]. This matches the same change already
made in the TypeScript, Python, Rust, and .NET SDKs.

Verification

• go build, go vet, and go test ./... clean; 12 new tests covering
  route + body shaping, filter omission, query escaping, the hydrated and
  literal-object edge shapes, and legacy-path compatibility.
• Verified live against app.memmesh.ai: 12,142 entities / 287,698 edges,
  decoding NVIDIA CORP -[reported_metric]-> Cost of Revenue and
  NVIDIA CORP -[ticker_symbol]-> NVDA.

Note

This branch is cut from origin/main and does not include the separate
uncommitted work sitting in the local checkout (brains.go, consent.go,
financial.go, pagination.go, and their tests). main builds fine without
it, so I left it alone rather than folding someone else's staged work into this
PR — but it is worth landing on its own.