Five registries disagree about which backends exist

[DROOdotFOO]

Five places independently declare which agent backends exist, and they disagree.
Each disagreement is a live bug class rather than untidiness, and adding a fourth
native backend widens all five at once.

Name	Resolver.@providers	Selector.@backend_table	ExecutorConfig.@type backend
grok_native	yes	yes	missing
cursor	missing	yes	yes
codex	no	reserved, intentional	yes

And in raxol_symphony, Runner.resolve_from_config/1 (runner.ex:99-115)
resolves "raxol_agent_session" and "noop", while
Config.Schema.@supported_runner_kinds (config/schema.ex:23) lists neither,
so both resolve and then fail their own preflight.

Consequences today

• grok_native is absent from the typespec, so Dialyzer cannot see it.
• cursor is absent from @providers. @by_string (resolver.ex:164) is
  derived from @providers, so harness_from_string/1 cannot name it: cursor
  is selectable in code and unreachable from /login, from --backend string
  resolution, and from .raxol/config.json.
• The docs drift the same way: AGENT_FRAMEWORK.md:206 omits grok_native and
  cursor, CODING_AGENT.md:93-94 omits all three native backends.

Proposed direction

Derive the tables from one declaration rather than merging them by hand, with
Resolver.@providers becoming a filter over the catalog so cursor is declared
and selectable rather than absent. ADR-0034 argues this should land alongside
the next name added rather than after it.

Recorded in docs/adr/0034-foreign-coding-agent-seams.md, gap 4.

[DROOdotFOO]

Already landed in ce696d7 (PR #971, merged 2026-09-06), which implements ADR-0034 Gap 4 exactly as proposed here:

• Raxol.Agent.Backend.Catalog (packages/raxol_agent/lib/raxol/agent/backend/catalog.ex) is the single declaration.
• Selector.@backend_table, Resolver.@providers/@by_string and ExecutorConfig.@type backend all derive from it at compile time, so cursor is now reachable from /login, --backend and .raxol/config.json, and grok_native is in the typespec.
• Symphony: Runner.@kinds generates resolve_from_config/1; Config.Schema.@supported_runner_kinds is pinned equal to Runner.configurable_kinds/0 by runner_kind_convention_test.exs.
• AGENT_FRAMEWORK.md and CODING_AGENT.md name the catalog.

Verified today on master: catalog_test.exs 9 passed, runner_kind_convention_test.exs 11 passed. Nothing left to do.