CLI-managed MCP server setup, without required env variables for credentials

Summary

Make setting up an MCP server easy from the CLI, and stop requiring an environment
variable for the credential-store key. Today both are manual and error-prone: adding
a server means hand-editing the operator settings.yaml mcp.servers[] block, and
the local OAuth credential store can only take its encryption key from a MECATL_*
env var that must be present in every shell/process that logs in or reads the credential.

Motivation

Writing the setup guide for connecting mecatui to an OAuth MCP server (the Stacklok
staging connector-gateway) surfaced two friction points:

1. No CLI to manage servers. mecated mcp only supports login
   (cmd/mecated/command.go). Adding a server means hand-writing a ~20-line nested
   mcp.servers[] OAuth/DCR block with several non-obvious, easy-to-get-wrong fields:
   the server name must be [A-Za-z0-9_]+ (no hyphens), url must include the
   resource path (e.g. /gw/mcp), issuer must be the exact origin,
   client: {mode: dcr, dcr: {}}, plus credentials and network blocks. A typo in
   any of these fails discovery with a generic, detail-suppressed error (the specific
   cause is dropped in internal/app/mcplogin.go's loginDiagnostic).

2. The credential-store key must come from an env var. The local OAuth credential
   store (MCPLocalCredentialProfile, internal/adapter/permconfig/schema.go) accepts
   only key_env: MECATL_*. So the operator has to generate a 32-byte key, store it,
   and export MECATL_MCP_CREDENTIAL_KEY in every shell that runs mecated mcp login
   and every process that later reads the credential (e.g. mecatui). Miss it in the
   runtime shell and the credential silently can't be decrypted → the server drops to
   "login required" with no obvious cause. This is exactly the "harder to set up in a
   durable way" concern raised in review.

Proposal

1. CLI management of MCP servers

Add mecated mcp subcommands to add/list/remove/inspect servers without hand-editing YAML:

• mecated mcp add <name> --url <url> --auth oauth — sensible OAuth/DCR defaults;
  discover issuer/resource from the server's RFC 9728 protected-resource metadata
  where possible (the same discoverDCRMetadata path login already runs), prompting
  only for what can't be discovered, and validating up front so it can surface the
  specific failure (issuer mismatch, DCR unsupported, missing scope) instead of the
  current generic "authorization unavailable".
• mecated mcp list / mecated mcp status — configured servers plus connection and
  credential state.
• mecated mcp remove <name>.

These read/write the same operator settings.yaml mcp.servers[] schema, so config
stays declarative and reviewable — the CLI is a safe editor over it, not a second
source of truth. mecated mcp login <name> already exists and composes with this.

2. Don't require an env variable for the credential-store key

Give the MCP local credential store the same key-custody choice the OIDC provider
credential store already has. NativeCredentialKey
(internal/adapter/permconfig/providers.go) already supports source: keyring | environment,
defaulting to the OS keyring. MCPLocalCredentialProfile should adopt the same union so
the default is keyring custody (macOS Keychain / Secret Service / etc.) with no env
var, and key_env becomes one opt-in option (for headless/CI) rather than the only
mechanism.

Result: mecated mcp login <name> and the runtime both get the key from the keyring
automatically — nothing to export, nothing to re-export per shell, and it survives
across sessions and processes durably.

Acceptance sketch

• mecated mcp add|list|remove manage mcp.servers[] entries in operator
  settings.yaml; round-trip preserves unknown top-level keys.
• The MCP local credential store defaults to keyring-backed key custody; no MECATL_*
  env var is required to log in or to read the credential at runtime. key_env remains
  supported for headless deployments.
• Docs updated to the CLI-first, no-env-var flow
  (user-docs/features/mcp-oauth-and-credentials.md, and the eng setup guide).

References

• Current CLI surface: cmd/mecated/command.go (only mcp login).
• Env-only key today: internal/adapter/permconfig/schema.go (MCPLocalCredentialProfile.KeyEnv).
• Existing keyring pattern to reuse: internal/adapter/permconfig/providers.go (NativeCredentialKey, source: keyring).
• Generic-error masking to improve alongside add: internal/app/mcplogin.go (loginDiagnostic).

Start with cmd/mecated/command.go for the existing mcp login surface, then read internal/adapter/permconfig/schema.go and providers.go to compare MCP and provider key custody. Review internal/app/mcplogin.go for diagnostic behavior and the referenced MCP documentation. Done means add/list/remove, keyring and key_env flows, YAML round-tripping, diagnostics, and docs meet the acceptance sketch.