lint: add 7 architectural-sync cops (catches 10 real bugs)

[dgageot]

Adds 7 new project-specific cops to the custom rubocop-go linter, focused on architectural sync invariants that golangci-lint cannot reach. Caught 10 real bugs along the way and locked in 5 architectural caches with documented justifications.

New cops

Cop	What it enforces	Real findings
Lint/ConfigLatestTagConsistency	json and yaml struct tags in pkg/config/latest/ agree on omitempty/omitzero. Frozen vN/ versions are exempt.	1 bug — Defer DeferConfig had json:"defer" but yaml:"defer,omitempty", so JSON always serialised the empty struct while YAML omitted it. Fixed with omitzero (consistent with the existing lifecycle.go pattern; DeferConfig.IsEmpty() makes omitzero semantically correct).
Lint/ConfigVersionsRegistered	pkg/config/versions.go calls Register for every pkg/config/vN/ and pkg/config/latest/ package on disk.	Preventive — currently in sync; locks the invariant.
Lint/TUIViewPurity	View() string methods on TUI models do not mutate the receiver (Bubble Tea purity). Slice-cache idioms (nil, [:0], append) are auto-exempt; intentional click-zone caches use //rubocop:disable Lint/TUIViewPurity (the //nolint prefix would collide with golangci-lint's nolintlint).	5 surfaced — 2 in tabbar.go (scroll-state mutations) + 3 in dialog click-zone caches; all annotated with one-line justifications.
Lint/RuntimeEventRegistry	Every &XxxEvent{Type: "yyy"} constructor in pkg/runtime/event.go has a matching entry in the decoder registry map in pkg/runtime/client.go.	3 bugs — MessageAddedEvent, ModelFallbackEvent, SubSessionCompletedEvent were silently dropped by remote-runtime clients (only a slog.Debug("invalid_type") log line). Now registered.
Lint/RuntimeSessionScoped	Every *Event struct with a SessionID field implements GetSessionID() string (the SessionScoped interface used by the persistence pipeline).	6 bugs — UserMessageEvent, StreamStartedEvent, StreamStoppedEvent, SessionTitleEvent, SessionSummaryEvent, SessionCompactionEvent. Without GetSessionID, the persistence-observer's sub-session filter if scoped, ok := event.(SessionScoped); ok && scoped.GetSessionID() != sess.ID short-circuits and lets sub-agent events leak into the parent's transcript. Now all six implement the method.
Lint/HookConfigSync	EventXxx constants in pkg/hooks/types.go and HooksConfig fields in pkg/config/latest/types.go stay 1:1, matched by snake-case wire string.	Preventive — currently in sync (22↔22); locks the invariant.
Lint/HookBuiltinsRegistered	Every exported const Foo = "wire" in pkg/hooks/builtins/*.go (excluding builtins.go and tests) appears as the first arg of a RegisterBuiltin(Foo, …) call in pkg/hooks/builtins/builtins.go.	Preventive — currently in sync (10 builtins); locks the invariant.

Why these cops

The codebase has several multi-file architectural invariants that the Go compiler cannot enforce:

• Config versions — list of pkg/config/vN/ directories ↔ versions.go registry
• Wire-format events — runtime event constructors ↔ remote-decoder registry
• Session ownership — SessionID field ↔ SessionScoped interface
• Hook coverage — EventXxx constants ↔ YAML schema fields ↔ in-process builtin registry
• Render purity — Bubble Tea View() is supposed to be a pure function of state

Each of these is a "two declarations that must agree" pattern. Forgetting one of the slots compiles cleanly and only blows up at runtime — usually as a silent drop, not an error. The cops fail the lint gate at PR time instead.

All seven cops follow the same idioms as the existing four (config_package_name.go, config_version_constant.go, config_version_import.go, latest_imports_predecessor.go): embed cop.Meta, expose a New<Cop>() constructor, implement Check(p *cop.Pass), report via p.Report(node, format, args...).

Bugs caught & fixed

pkg/config/latest/types.go         Defer DeferConfig: json missing omitzero
pkg/runtime/client.go              registry missing 3 events
pkg/runtime/event.go               6 events missing GetSessionID

Cumulative state

63377309a lint: add two hook-architecture sync cops
92d1a6498 lint: add two runtime-architecture cops (catches 9 bugs)
ec5b3ce66 lint: add three project-specific cops              (catches 1 bug)
e5a4b67cd lint: adopt new rubocop-go API                     (already in main)

11 cops total, 10 real bugs caught & fixed, 5 architectural caches documented, 3 pure preventive invariants locked in.

Validation

• mise lint ✅ (golangci-lint, 11 custom cops, go mod tidy --diff)
• mise test ✅ (105 packages, 0 failures)
• Each cop verified to fire on a seeded violation and to go silent again when restored — including bidirectional sync cops (3 drift scenarios for HookConfigSync).