refactor(mcp): parseAddArgs via stdlib flag.FlagSet

[localai-bot]

Summary

Internal refactor of nib mcp add's flag parsing (cmd/mcpmanage.go's parseAddArgs). No behavior change.

The function grew flag-by-flag across the MCP CLI work and the recent remote-auth work (#39) into ten near-identical switch cases — two per flag (--url, --transport, --env, --token, --header), one for the space-separated form and one for inline --flag=value — plus a manual pre-scan loop to find a standalone -- and split off the stdio command's own argv.

Replaced with stdlib flag.FlagSet, mirroring the existing parseInstallArgs pattern already in cmd/plugin.go (two-pass flag.NewFlagSet(..., flag.ContinueOnError) parse, handling a positional <name> argument that can appear relative to flags). This isn't a new dependency or a foreign idiom — flag is already imported in cmd/mcp.go, cmd/plugin.go, and cmd/skill.go.

• --url/--transport/--token become fs.StringVar calls.
• --env/--header (both repeatable KEY=VALUE flags) share one small kvFlag type implementing flag.Value, registered twice.
• The manual ---boundary scan is deleted entirely: stdlib flag.FlagSet already terminates parsing at a bare --, strips it, and returns everything after it via fs.Args() — exactly the "rest is the stdio command's own argv" behavior the old code hand-rolled.
• Post-parse validation (transport must be http/sse, exactly one of command/URL) is unchanged.

Error message text changes in a few places (stdlib's own wording for missing-value/unknown-flag cases) — no test asserts exact text, only err == nil/err != nil, and this is a dev CLI with no message-stability contract.

Design spec: docs/superpowers/specs/2026-07-01-mcpmanage-flag-parsing-refactor.md (local-only, gitignored, not part of this diff)
Implementation plan: docs/superpowers/plans/2026-07-01-mcpmanage-flag-parsing-refactor.md (same)

Test plan

• All 10 existing tests in cmd/mcpmanage_test.go pass unchanged, both before (baseline) and after the refactor — no test file edits
• go build ./..., go vet ./..., go test ./... all clean across every package
• Manually traced the two highest-risk equivalence cases: the ---boundary termination (confirmed fs.Args() correctly excludes -- and returns the literal command argv, including a flag-shaped literal like -x), and the kvFlag map-aliasing (confirmed map[string]string → kvFlag conversion shares the backing map, so Set calls correctly populate srv.Env/srv.Headers)

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