fix: resolve Go vet errors

[beausterling]

Fix Go vet errors in authentication and self-install

Status: Ready for review
Scope: Two localized error-construction fixes
Behavior change: Error paths now preserve the correct underlying error and pass Go vet
Risk: Low; success paths and public command interfaces are unchanged

Human summary

This PR fixes two existing Go vet failures that prevent a clean go test ./... run. One error path wrapped a directory string instead of the actual write error, and another passed runtime-generated text to fmt.Errorf as though it were a format string.

After these corrections, the full test suite, vet pass, and both command builds complete successfully.

Problem

The repository's baseline validation failed with:

internal/cli/auth.go:367:23: non-constant format string in call to fmt.Errorf
internal/cli/install.go:44:46: fmt.Errorf format %w has arg binDir of wrong type string

These failures affected contributor confidence and prevented standard Go validation from completing cleanly.

Root cause

internal/cli/install.go

The self-install write failure used:

fmt.Errorf("writing binary to %s: %w", target, binDir)

%w requires an error, but binDir is a string. The actual os.WriteFile error was available as err and was being discarded.

internal/cli/auth.go

The HAR importer assembled a user-facing error message dynamically and returned it through:

fmt.Errorf(msg)

Go vet flags a non-constant format string here. No formatting operation is needed after the message has been constructed.

Changes

File	Change	Result
internal/cli/install.go	Pass err to %w	Preserves the underlying filesystem error for inspection and unwrapping
internal/cli/auth.go	Return dynamic message with errors.New(msg)	Preserves the exact message without treating runtime text as a format string

Both changed sites include // PATCH: annotations according to the generated-tree contribution guidance in AGENTS.md.

Behavior matrix

Scenario	Before	After
self-install binary write succeeds	Success	Unchanged
self-install binary write fails	Incorrect formatting argument; underlying error lost	Target path plus wrapped filesystem error
HAR import finds credentials	Success	Unchanged
HAR import receives empty/invalid HAR	Dynamic message passed as format string	Same dynamic message returned safely

Validation evidence

Executed successfully on macOS ARM64 with Go 1.26.4:

go test ./...
go vet ./...
go build ./cmd/skool-pp-cli ./cmd/skool-pp-mcp

Observed result:

• all packages built
• all existing tests passed
• go vet ./... exited successfully
• both CLI and MCP entry points built successfully

Security and privacy

• No authentication values, HAR content, cookies, or configuration files are included.
• Credential handling behavior is unchanged.
• The install failure path is improved because it now retains the real filesystem error rather than discarding it.

Compatibility

• No command names, flags, output schemas, API paths, or persisted formats changed.
• No new runtime dependencies were added.
• Success-path behavior is unchanged.

Review guide

Recommended review order:

1. Confirm %w now receives the os.WriteFile error in internal/cli/install.go.
2. Confirm the completed HAR error message is returned with errors.New in internal/cli/auth.go.
3. Verify the validation commands above.

Dependencies

None. This PR is independently mergeable.