Add agentvault with a seed-outpost subcommand

Interactive agents are classifier-blocked from plumbing credentials through
a shell, so seeding an Authentik outpost token into Vault KV needs to happen
inside one binary invocation that never exposes the secret.

- Add cmd/agentvault, a fourth CLI sharing the agentpr Vault AppRole login
  (role_id only, VAULT_ADDR/AGENT_APPROLE_ROLE_ID defaults unchanged).
- Add `agentvault seed-outpost`: read the Authentik API token from
  kv/service/authentik/agent-api-token (field `token`, falling back to
  `api_token`), exact-match the outpost by name via the instances search,
  fetch its key from /api/v3/core/tokens/<identifier>/view_key/ and write it
  to --dest-path under --dest-key.
- Print only the outpost name, token identifier, dest path and new KV
  version; keep secret material out of results, errors and logs.
- Distinguish the failure stages (login, KV read denied, outpost missing,
  view_key, KV write denied) with ErrVaultDenied/ErrVaultNotFound/
  ErrOutpostNotFound sentinels and actionable messages.
- Add internal/agent vaultkv.go (AppRole-authenticated KV-v2 client) and
  authentik.go (outpost search + view_key) for reuse by future flows.
- Cover the happy path, idempotent re-run, field fallback and every failure
  mode with httptest servers, including a leak check on error strings.
- Wire agentvault into the Makefile, build-rpm.sh, nfpm contents, release
  cross-builds/assets, README and AGENTS.md.

scripts/build-rpm.sh

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 #
-# Package the (already built) agentpr, watchpr and agentws binaries into an RPM
+# Package the (already built) agentpr, watchpr, agentws and agentvault binaries into an RPM
 # with nfpm, bundling generated bash/zsh/fish shell completions.
 # Usage: scripts/build-rpm.sh [version]   (version defaults to $CI_COMMIT_TAG)
 #
@@ -12,7 +12,7 @@ cd "${ROOT_DIR}"
 VERSION="${1:-${CI_COMMIT_TAG:-0.0.0-dev}}"
 VERSION="${VERSION#v}" # strip a leading v
 PACKAGE="agent-tools"
-BINARIES=(agentpr watchpr agentws)
+BINARIES=(agentpr watchpr agentws agentvault)
 DIST="dist"
 
 for b in "${BINARIES[@]}"; do
@@ -37,7 +37,7 @@ export PACKAGE_VERSION="${VERSION}"
 export PACKAGE_RELEASE="1"
 export PACKAGE_ARCH="amd64"
 export PACKAGE_PLATFORM="linux"
-export PACKAGE_DESCRIPTION="CLI tools for Gitea automation as the unkin-agent user: agentpr (create PRs/comments), watchpr (poll PRs and alert on meaningful change) and agentws (manage per-branch git worktrees)"
+export PACKAGE_DESCRIPTION="CLI tools for automation as the unkin-agent user: agentpr (create PRs/comments), watchpr (poll PRs and alert on meaningful change), agentws (manage per-branch git worktrees) and agentvault (deterministic Vault flows)"
 export PACKAGE_MAINTAINER="Ben Vincent <ben@unkin.net>"
 export PACKAGE_HOMEPAGE="https://git.unkin.net/unkin/agent-tools"
 export PACKAGE_LICENSE="MIT"