mavend: group the recall fields into one wiring struct (V-433)

The review comment asked for basic DI. The answer is the idiom voice.go
already had for capabilities — a cohesive *Wiring struct — applied to a
group that is not a capability toggle, plus the decision written down so
it is a rule and not a habit.

recallWiring holds the embedder, the vector store, the personal boundary
and the two numbers that gate an answer. They sat in three places on
reactiveHandler, with the gate numbers a hundred lines from the store
they gate. Its zero value means no recall, so it is a value, not a
pointer like the optional-capability groups.

dataStore stays out of it. patterns.go, ecosystem_acts.go and confirm.go
use it, so it is not part of this cluster.

docs/handler-wiring.md records the choice, rejects a container or a
wire-style generator outright, defers narrow per-handler interfaces to
the package split that would justify them, and states the constraint the
task named: a wiring change does not ride a feature PR.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/handler-wiring.md

@@ -0,0 +1,62 @@
+# How reactiveHandler is wired
+
+*Last verified: 2026-08-04 @ b6abb19. Living doc: correct it in place, do not append.*
+
+The decision Vikunja #433 asked for, and the rule that follows from it.
+
+## The decision
+
+**Group the fields into cohesive wiring structs. No container, no generator, no
+framework.** `reactiveHandler` (`cmd/mavend/voice.go`) stays the one type the voice
+server talks to. What changes is that a capability arrives as one named group, not as
+four more loose fields on a struct that already had thirty.
+
+The pattern was already in the file before this was written down: `searchWiring`,
+`kiwixWiring`, `homeWiring`, `netWiring` and `ecosystemWiring` are all this shape, each
+`nil` when the capability is off. #433 makes it the rule rather than a habit, and adds
+the case the habit had missed — a group that is not a capability toggle.
+
+## The worked example
+
+`recallWiring` (`cmd/mavend/recall.go`) holds the five things the recall path needs:
+the embedder, the vector store, the personal boundary, and the score and margin that
+gate an answer. They used to sit in three separate places on the handler with the two
+gate numbers a hundred lines away from the store they gate.
+
+Its zero value means "no recall", which is why it is a value and not a pointer. The
+`*Wiring` types that model an optional capability stay pointers, because `nil` is how
+"not configured" is spelled and a zero-valued search client would be a client pointed at
+nothing.
+
+## Why not the alternatives
+
+**Narrow consumer-side interfaces at each handler** is the more idiomatic Go answer and
+it is not rejected, only deferred. It is the right move at the point a handler is pulled
+into its own package, because that is when the import direction starts to matter. Doing
+it first would mean writing an interface per handler against a struct nobody can pass
+anywhere, which is churn bought against a package split that has not happened.
+
+**A container or a wire-style generator** is rejected outright. This is one binary with
+one composition root (`wireVoice` in `cmd/mavend/voicewire.go`). Generated wiring would
+add a build step and a layer of indirection to solve a problem that is currently one
+composite literal long, and it would make the "is this capability configured" question
+harder to answer by reading, which is the question this file is mostly about.
+
+## What this unlocks
+
+The reason `cmd/mavend/` cannot split into `mavend/actions/` today is that every action
+handler is a method on a struct with thirty unexported fields: moving handlers to a
+subdirectory means exporting all of them or inventing an interface to pass through. That
+was the answer given on the tick.go and voice.go splits, and it is still true. Grouping
+is the step that makes it false later — a handler that takes `recallWiring` and nothing
+else can move without the other twenty-five fields following it.
+
+## The rule
+
+**A wiring change does not ride a feature PR.** The voice.go and tick.go splits were
+safe to merge because the moved code diffed identical, line for line. A regrouping that
+touches the confirm gate or the act allowlist is its own change, reviewed on its own, or
+it is not reviewable at all.
+
+New capability, new group. A capability that adds four fields to `reactiveHandler`
+instead of one struct is the thing this decision exists to stop.